Starcraft supports only .wav files in the map editor when you want to make sounds play in-game. Since players must download maps before playing them, it is important to make these files smaller, but if you wish, still have a bit of quality within them.
Mono vs Stereo
When compressing a sound using whichever program you wish (windows sound recorder works well for this, but audacity works good as well), you'll be given the option to make a sound to mono. The answer to this, is yes.
It is standard that TVs and computers have two speakers, one for the left and one for the right. In certain situations, games or movies will want a character to talk on the left, so to make it sound like its coming from that direction, the sound is coming from the left individual speaker. Stereo allows for this to happen, to map each speaker individually. For StarCraft, it is just a waste of space, since it is an RTS game and the experience doesn't change at all from what direction the sound comes in. Converting the sound to mono will remove the second channel, thus cutting the file size in half, and all it does is output the same sound for both speakers, much like you'd be doing with stereo anyway.
16-bit vs 8-bit
The difference between the two is that 16bit allows for higher quality sounds, if you don't set the sampling rate all the way to the bottom. While Staredit can compress 16-bit sound files, a little experimentation with sounds of different bit rates saved into a map reveals that 16-bit sound files take up less space, so 16-bit sounds are your best option.
This is the important one that you'll have to play around with. Sampling rate reflects on the quality of sound you hear. If you set this all the way to the lowest value possible, then the sound quality will be very poor, but take up the least amount of space in your map. It is important to balance quality vs file size, as music will sound better with slightly higher sampling rate, and most sound effects won't sound any different at the lowest sampling rate.