That is almost a general question, but from here on it becomes highly opinionated.
I’m talking about the bands that march past my house every few months or so. There is one marching past my front doorstep as I type there words. From the sound of it, I am sure that somebody must be stomping out a bagpipes fire somewhere in the middle of the parade.
Often the band is from one of the local high schools, and I can usually reconcile the flat notes & lack of coordination to the assumption that they are students who have been studying the french horn for a few semesters at most. “Aww that’s cute”, I say, “There’s a bunch of kids all trying to play the french horn at the same time”. But this parade I am looking at is full of mature adults in their 30’s, 40’s & maybe even 50’s. Surely they didn’t just pick up a trombone a few months ago and decide to start learning how to play it.
Maybe it’s because there is always an overabundance of brass & percussion, and no matter how skilled the musicians were all marching bands would still sound like that. This band sounds like 60% brass and 75% percussion. And yes I realize that is mathematically impossible but believe if you heard this band, I bet you’d agree.
I say we need to get a piano worked in some how. I know there are certain pratical problems involved with having a piano in a marching band, but nothing a flat bed pickup truck couldn’t fix.
And why is the cymbal crash necessary precicely every 7 seconds by my watch? And why so many people playing snare drums? Is it really necessary to have 20 people playing the same kind of drum? No wonder they can’t keep the same rhythm. And what in hell is that song they’re playing anyway? Are they plaing it so badly that I can’t even recognize it or is it just a song nobody has ever heard before? You’d think it would be something fairly popular for them to be marching down the street playing for the entire neighborhood.
Has anybody ever seen a marching band with a large contingent of banjo players? That would probably still sound awful, but at least it would be different.
I can think of two possibilities: either they just aren’t very good, or - if you’re standing right there as they pass by - you’re only hearing one section at a time instead of the entire band because you’re too close to them. If you saw them play in a football stadium they’d probably sound much better.
Unless, of course, my first suggestion is the truth.
I’m a marching band veteran, so this is an easy one.
Parades will always sound horrible as the band is right in front of you, because as KWM mentioned, you’re only hearing one section at time. Actually, more precisely, you still hear all the sections, but not balanced the way they should be.
Other possibilities include:
If the band members are older, they may not play very often, or perhaps they haven’t played in 30 years.
Often there is an overabundance of brass and percussion–but that, in itself, will not make the band sound bad.
Marching, especially if it’s real marching where the members of the band lift their feet off the ground an appreciable distance, and not the drum corps style that most high school directors are sticking the students with, can be jarring. The mouthpieces can jiggle on the players’ mouths if they aren’t marching exactly correctly, and this obviously creates less-than-optimal conditions. I’ve been playing trombone for 12 or 13 years, have taken private lessons for some time, and can tackle all but the most difficult college-level concert pieces with little effort (enough tooting my own horn, har har), but marching still makes me sound like I haven’t been playing long if I’m having a day that’s less than perfect.
All that aside, a marching band will sound somewhat better on a football field, most assuredly. It will sound incredibly better during a “feature” number where the band doesn’t move. All this still won’t sound incredible unless you’re DCI (Drum Corps International) material, though, because of the high volumes needed on a football field to be heard. Loud often means loss of tone quality.
Enough out of me. 
To add to the above, people can detect a delay in sounds that travels 30 feet or more (more or less.)
While an orchestra is seated fairly closely together in an auditorium, marching bands are usually spread out across the field or street. Because of the distance, the musicians hear (and play) the beat at different times.
Even if a marching band was perfectly in time, the musicians farthest from you would sound like they were behind the beat.
Apparently you don’t live anywhere near Philadelphia, or you’d already be familiar with the String Bands contingent of the Mummers’ Parade on New Year’s Day.
As far as I can remember the “strings” are entirely banjos. There are other instruments as well, but the centerpiece is a truly frightening number of banjos.
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I have NEVER heard a marching band that sounded like it was worth HALF of the expense and energy that went into forming it, with the following exception:
The Royal Canadian Air, Army and Sea Cadets.
I can’t stand those cheesy civvie bands with their gaudy feather-and-sequin uniforms and their eight-year-old girls in skirts twirling flags. Usually, they can’t play, they can’t march, and they have all the professionalism and politeness of a gaggle of geese.
But take a listen to those magnificent cadets… if the Canadian Cadet Tattoo ever comes to town, GO. Cadet bands can be good, they can be bad, they can be great; but they’re miles ahead of those stuffed shirt private bands.
The best part? These cadet bands are composed of 12-18 year old kids who do this in their spare time, among the demands of school and life and cadet training. Way to go Cadets!
[Particularly impressive: 200 Wolf Squadron RCAC / 134 RCSCC Admiral Mountbatten / Central Region Mass Band / etc]