I was looking to get a starter library to start practicing sound editing until I can afford my field recording setup. Blastwave fx has a starter library for $40 but its all in mp3 format. It has 500 sounds. Are mp3 format sounds ever used in commercial film or games? Or should i just use it as a practice library only?
Over and above all the great answers above, there is one important aspect... training your ears. If you spend all this time practicing on MP3s, you'll get used to that and it will be your standard. Don't you want to train your ears on "higher quality" material? I'd think it's part of the training process.
Just a thought.
In broadcast, mp3 should only ever be used as an absolute last resort. Like, the original wax cylinder recording shattered, every LP copy in existence has melted, the original master tapes were all de-magnetized, the gold master CD and all subsequent dupes were run through a shredder, and every hard disk with the .wav file has had a run-in with a set of bolt cutters. Of course, having said that I still get music delivered in OMFs as an MP3 probably once every 4-5 shows. To their credit, they're at least high quality mp3s. :)
I think in actuality it depends on your destination. If what you're designing is only going to be played on the web, it'll probably only ever be heard as an mp3 anyway. While there's the argument that you should always start with the highest quality possible and use that to reach the lowest common denominator, if the highest quality you can afford is $40 for 500 mp3s, so be it. Download away and get to practicing. But, if you pack your lunch for a month you could pick up an H1 for $99. And that has the potential for way more sounds, and a little learning along the way.
MP3 arent used in films - many is the time a picture editor has cut in temp music using MP3s and they sound dreadful up big (and are always replaced with uncompressed) - its a bit like how a JPEG image looks ok on a small screen but the grain is apparent when projected larger... The grain/compression will also become apparent when you start manipulating the sounds eg pitching etc... if its just for practice go to freesound
With sound effects in MP3 you will first be sacrificing transients (did you know MP3 runs at roughly 40 frames per second?). Due to its limited bandwidth, spectrally rich sounds will lose a lot of their frequency content in MP3, leaving you only with what the algorithms have judged you would pay attention to. Of course, after any processing, you can throw all of this advance judgement away.
Having said this, foley and small/spot SFX might be OK when MP3d, since it was designed for full-spectrum music, but these will sound a bit smeared, and further processing will quickly bring up all sorts of artefacts. Anything delicate, or sounding rich, or with noise components, forget it. Any ambience - prohibitive in MP3.
I wish more people actually looked into how this kind of compression works, and that there are better schemes on the market. Even M4A has more temporal resolution but to really hear what you're doing to your audio, compress something as OGG and then re-compress the result as MP3.
Still, trust your ears.
Console games can use mp3 as final playback, but it is not the most common and also can be quite tricky to use. Also if you use mp3 as a source file, the game engine/middleware would in most cases convert your mp3 to PCM and back to mp3 which isn't the best thing to do. 44.1khz 16bit is minimum standard for library material imo. like Tim said - if you are just looking for free noise freesound.org is great. Also some of the indy libraries have free samples.
If all you're looking to do is practice, and your not going to use these things you do for any commercial purposes, you may be better off just heading over to freesound.org. It's a creative commons audio library. I'd avoid using them for anything commercial (or even non-commercial public) though, as one of the stipulations is that you have to credit EVERY author of sounds your using. I also think there's a provision that you can't use originals (just derivatives). Not very useful for projects you want to share with the world, but for at home experimentation it should suit your needs very nicely.
That way, you can save your money and put it towards a decent library in the future.