That brings back memories!
Anyone who used them would be familiar with the sound - you would hear it coming through the computer when you loaded a program from a cassette. It was a sreechy sound similar to the sound of a dial-up modem.
If you put it in a cassette player which went to a set of speakers rather than a computer it didn’t sound much different, though if you put a music cassette into a player that went to a computer you could hear the music, but it was tinny (I’m guessing the computer cut out the lower frequencies) and fairly quiet.
Some cassettes had both computer games and music on them. I remember we had the Band Aid compilation of computer games, which also had “Feed the World” on one of the sides of one of the tapes, which would play through our ZX Spectrum if you left the tape player running after the game on that side of the cassette had loaded.
To (partially) answer the other bit of your question - You could use the cassette player port on your computer to play music of sorts. I had a couple of TRS-80 games that had rudimentary sound effects if you connected an amplifier to the tape-out socket. Similarly I had a ZX81 program that output music tones in the same way. Neither of these computers had any sound chips by the way, the noise was made by programming the cassette-out port directly rather than a dedicated sound generator.
I remember saving programs to cassette on a Commodore Datasette tape drive. If you played the tape in an audio system, it mostly sounded like buzzing at around 120 Hz, with some higher frequency noise superimposed on that.
Somebody actually posted a movie of the Datasette sound. my memory is of the portion from 0:03 to 0:06, but there’s a lot of other different sounds going on before and after that.
That’s the sound I remember.
In the 1970s and 1980s, there were radio and TV shows in which they distributed software by playing the software (i. e. the audio signal) on the air. Computer owners recorded the sound on audio cassettes and loaded the software onto their personal computers. Of course, this sounded horrible and I remember one TV show which played the noise during the closing credits.
This is mind blowing.
And I thought phone-phreaking by playing back punch tones was amazing.
ETA: Speaking of TV, no reason why software can’t be distributed with steganography, right?
NSA no doubt way ahead on that.
During my senior year of high school (1982-83), we had a Computer Science class, in which we programmed on Radio Shack TRS-80 computers with cassette tape drives.
A friend of mine (who was taking the class with me) came to pick me up for a D&D game in his car. He had a cassette (his storage cassette from the class), which he’d labeled “HAL’s Greatest Hits”. “Here, you gotta listen to this”, he said, as he stuck it into the car’s tape deck. I seem to recall there was some variety in the sound that came out of the speakers, but yeah, just unintelligible noise.
I recall a few more beeps and boops at the beginning of my programs, but otherwise, yeah, that’s pretty much it.
I had a ZX Spectrum (my second computer) - you could hear the noise of the data being loaded - and the bitrate was sufficiently slow that you could in some cases guess at what was actually coming in (only in a general sense) - many programs loaded the whole memory starting with the video graphics area - and solid blocks or patterns here made the sound of the data quite distinctive.
The standard loading routine (as distinct from custom/higher speed loading routines that some software later implemented) sounded quite like a fax machine handshake.
I had a cassette program, I want to say Atari Lightshow on the 800, that would play an audio track through the cassette drive’s internal speaker while the program was loading. The program data was inaudible, and when the load was finished a voice on the tape would tell you to stop the cassette.
Maybe the Spectrum had this problem, but Commodore Datasette was completely silent when reading from a tape. Usually the only way you could hear what was on the tape was to play it in a regular audio tape player. However, I once downloaded a Commodore 64 demo which fed the Datasette’s output to the machine’s SID (sound) chip, so you could listen to whatever was on the tape—including music—through your computer’s external speakers. Of course, this wasn’t very practical, since the sound quality was terrible, and the computer couldn’t do anything else while playing the tape. It was just a clever hack.
I remember some of the data tapes came with a warning not to play them on audio equipment as you may damage it (it=the tape or player, I think it was the player)? Anyone recall anything like that?
You are, of course, familiar with the Timex Data Watch? The article talks about computers, but it worked the same way with broadcast TV.
If played at sufficient volume, the noise could damage a speaker system specifically designed to be damaged by noise from a data tape. The warning was in the nature of a legal disclamer.
When I got to the part in learning programming (in BASIC) on the TRS-80 where it covered music and sounds, I couldn’t afford an amplifier, so I had to record the sounds I was making, then unplug the recorder and playback the tape to see how it sounded.
I had a TRS-80 Model I we got around 1978. Almost immediately took it back to get the upgrade that quadrupled its RAM. 4k was tight, but 16k was luxurious.
Yes boys and girls, my first computer’s memory was 1/250th of a megabyte. 
I can vouch for the fact that a cassette tape that’s gone through a cat will not play any recognizable sound.
While my computer never shared the noise of programs with me, I became quite familiar with it.
Rather than try to make special products for very limited markets, companies that sold software would make one tape that had the program for several different computers on it. So to load the program for … Midway Campaign for the TRS-80, you’d have to fast-forward past the version for the Atari 400 (The reverse side of the tape had the Commodore PET version and something else).*
So you would have to unplug the player/recorder, then hold down both fast forward and play (which they told you NEVER to do) so you could hear when you got to the end of the first file. Then stop, plug everything back in, and press play.
*These may not be exactly factually correct. I’m not sure what order the files were in for Midway Campaign, and sometimes TRS-80 was first, or on the B side of the tape. Most stuff from Avalon Hill had files for 4 different computers and I can’t remember what the fourth one was, most stuff from Radio Shack had versions for the Model I, Model II, Model III and so on.
Must have been nice to have such a huge amount of memory! : )
My first computer was one of these with a whopping 2K of RAM:
http://en.wikipedia.org/wiki/Timex_Sinclair_1000
I later had one of the Datasettes for the VIC-20 that Machine Elf linked to above.
Before that, for both computers, I spent quite a lot of time manually writing down my programs when I was done for the day and re-typing them back in the next chance I got to work on it.
And you wouldn’t believe how steep the hill was that I had to climb both to and from school every day!
You mean like fired as a projectile through the cat or more like the digestive route?
Either way, LOL wut?!