If I understand correctly, the “App Center” is the Snap Store. But the terminal commands I mentioned should uninstall the snap and install the deb. As for a version mismatch, when you run Audacity, under Help → About Audacity it tells you the version number.
The aup3 file that i saved under the other version of audacity. 
Yes, this is version 3.4.2, which is quite old. They are up to 3.7.x
Makes sense that 3.4.2 balks at opening a 3.7.x project file. I would actually try next (yes I am testing it right now) to do what @orcenio says and run the Appimage https://github.com/audacity/audacity/releases/download/Audacity-3.7.7/audacity-linux-3.7.7-x64-22.04.AppImage
I Downloaded the Appimage in the link, enabled the executable permission (from the GUI file browser you can right-click and “allow executing file”), ran it; it asked me about generating a UUID (which I refused), cloud storage (I clicked on the x to close the window), then it ran as usual and outputted sound successfully.
NB you are evidently using Ubuntu 24.04 LTS but I tested under 25.10 — the same AppImage should work, however
Most likely that version is 2.4.2 and not 3.4.2. It looks like Audacity follows the standard major.minor.patch type of semantic versioning, so that is quite a bit older.
Cool. Follow DPRK’s instructions and let us know how you fared.
ETA: Nah I’m wrong. You are on 24.04LTS. Not 22.04LTS (silly me). That means Audacity is this one 3.4.2 (It would have been odd for you have an Ubuntu version that is from 2022, ay)
I asked my husband for help. He read through the instructions on ubuntu’s and audacity’s sites. The most recent “standard” version of audacity on this version of ubuntu is quite old. I needed to install a backport (new word to me) to get a more recent one. Ubuntu does not install backports by default. He found that the most recent backport (noble backports) was probably recent enough, and also found the right magic words to install it. I typed in my password. 
I now have Audacity 3.7.3, which is recent enough to share a file type with my other versions. I’ve yet to see if I made any progress in using bluetooth. To be continued…
Ooh, now i have a choice of 16 different playback devices. They have user-friendly names like the aforementioned sof-hda-dsp: - (he:0,0), and also “default”, “sysdefault”, “samplerate”, and “pipewire”. (And 12 others)
…
OMG! When i connect my headphones, tell it to rescan audio, and play, it played through the headphones! Success!
I’m going to guess the apt rather than snap did the trick.
The AppImage is compatible with 22.04 but it works under newer versions— there is not even a specific 24.04 AppImage available yet.
P.S. If anyone reading this is confused, an “AppImage” is not a package per se—you just run it like an executable.
I am glad it worked out! I blame snap permissions under Ubuntu 
But, it goes to show that it can be difficult to diagnose “Linux” problems. I have had analogous issues that wasted quite a bit of time.
Nice. Each troubleshooting is a journey in learning about your system. Your comfort level and general capabilities will grow if you are willing to suffer through these set backs.
Oh yes. I was seeing that and also getting confused as to what version puzzlegal had. too many numbers at once for me.
These last rounds of conversation have certainly not been encouraging for any novice contemplating a switch to Linux (which I was not, and even less so now). But my oldest laptop is basically now a throw-away – 2 GB, 128 GB SSD, AMD Sempron processor. I thought it might be fun just to install Linux Mint on it based on previous recommendations, but even that may not be possible becaue it might be a 32-bit processor which I understand Mint doesn’t support. I’d have to check the specific model because many Sempron chips were 64-bit, but to hell with it. It would be a hobby like building a Lego kit. My Windows computers run just fine, even if Windows 11 is a bit annoying.
I am not going to sit here and argue you should try Ubuntu 
They have indeed dropped that support from Linux Mint. Try (e.g.) MX Linux, or regular Debian. KDE is a decent desktop interface.
AMD Sempron added amd64 support in 2005, apparently.
Even Debian “the Universal Operating System” has dropped 32 support now (which is probably why all the Debian based distros Ubuntu/Mint/etc followed suit). When the Debian community drops it, it is pretty much done.
He’ll need to take drastic measures and use one of the lightweight distros (like Alpine Linux) designed to run on a paperclip.
ETA: or as you suggest try a non-linux OS like BSD. Which would be interesting.
The slogan is “Yes, it can run NetBSD”, not “Yes, it can run Debian”…
Just wanna drop in to say: I’ve been running Kubuntu for years. I’ve never heard of the sneaky Snap installs talked about above. I just checked with “snap list” in the terminal. Other than system stuff, the only thing there is Firefox. Which is interesting, because I uninstalled the Snap version a long time ago, and properly added the Debain version and its repository. Not that it matters; I rarely use Firefox anymore, but when I do, I know it’s the “correct” version running.
There is also Arch Linux 32 (never tried that one; I have tried the official Arch Linux)
Please don’t try to install Linux.
But as i said above, this is basically the same issue i had with my corporate Windows 11 laptop a week or two ago, and was resolved in basically the same way, except that linux command line expertise is much more widespread and generous than Windows command line expertise.
Also, while i still need to check and see if i need to install LAME on this version, it’s currently running better than either my Mac or my windows version of audacity. Both of those are incredibly fussy about audio input, and need to be restarted if i change the options. (Such as by attaching headphones.) This is the first time I’ve ever had that “rescan audio” feature actually work.
Yeah, I’m pretty sure that was the issue.
Geeze… I don’t often say this, but I think that is literally a throwaway… you could probably calculate faster on an abacus than that thing… 
You might as well put Windows 3.1 back on it!
On the other hand, if it still has a COM port or IrDA or similar, it might be fun to use it to tinker with other legacy devices…
You might be surprised. I mean, it literally is a throw-away now, but it came with Windows XP (which I chose over Vista) and then I doubled the memory and installed Windows 7. Windows 7 SP1 runs on it just fine. Back in the day it was a very useful sandbox for testing and transitioning my applications from XP to Win 7.