On making "nice" MP3s:
I've made quite a few commercial CDs down the years and have proper editing software, including a Sony MP3 codec: it's very sensitive to 'signal level'*.
I think they all are. If the level is too low, you get quite unpleasant artefacts. If you get it as loud as possible, the MP3 codec makes better use of its very limited dynamic range - you can reduce the bitrate quite significantly and keep acceptable quality (for really c**p values of 'acceptable').
If you're just using something like Nero and drag-dropping tracks, you may not be able to do this, but if you're using some sort of audio editor to assemble the tracks together...
... normalize to between 1dB and 0.5dB below maximum (depends how brave you're feeling). In my experience, you get reasonable speech results at 22kHz sampling. Music will be tricky, depending on style and content - don't expect tiny wonderfulness with anything that has a lot of long synth notes or classical music!
If you want to get the smallest acceptable filesizes, make the file mono, then normalize (you have to do this in the right order!). For most PA-type applications, including party music, stereo is usually unnecessary.
Hope it helps. To my ear MP3s rarely sound other than nasty - acceptable for talking books on the bus, etc. but not much else. I did serious and very expensive damage to some speakers a few years ago because someone partied with them and fed them distorted MP3s (blew up some almost unobtainable tweeters).
If in doubt stick to wav files - they can be uncompressed, like CDs, and will be a lot cleaner.
E.
Aside: One long-term annoyance is that the 'Red Book' standard for the CD format mentions a quadraphonic implementation (never used AFAIK), and the normal stereo, but not mono. If they'd made slightly more effort the things would've been so much more useful, with roughly two hours playing time! Even now, although you can put totally unrelated material on the two channels (there's no crosstalk as it's digital), you can't define the track times independently. It's really annoying, as most language courses would have fitted on one disc!
