uLAW files

It's a heavily compressed non-linear format used for things like telephone messages, etc. .sesx isn't an audio format (it's just session file information) so we'll ignore that. To save a wav file in this format you use Save As, and select Libsndfile as the format. Then you go to Format settings and select Sun/NeXT AU format, and then underneath that in Encoding, select uLaw 8-bit PCM (ITU-T G.711). If you think that's a bit of a pain, the next thing I have to warn you about is what saving to that format will do to your sound, and what you have to do about it.

Fundamentally this format sounds somewhat worse than old cassette recorders, if you remember them. Being an 8-bit format it's going to introduce a lot of noise, and there's absolutely nothing you can do about it - it's the Laws of Physics at work (6.02dB/bit means a noise floor of about -48dB). So to make everything remain audible, it has to be compressed so that it remains nearer to 0dB than normally you'd be comfortable with. Unless you want to damage peoples' ears, this format is only suitable for speech; music generally sounds absolutely awful on it.

I know it sounds a bit strange, but the higher the quality of your original file, the less it will degrade when it's converted. If you're going to produce crap, then make sure that it's the highest quality crap you can make it! Seriously though, you may well have to do some work to your file first.