Good point. Even though they're only a second or two long, they're huge files (several hundred kilobytes each) and I have no idea how to reduce them. They're like 10 times the size of the standard Glest *.wav files.
EDIT: Disregard that. I'm an idiot. The problem wasn't that it couldn't open it because it was an *.ogg, but rather because I made a typo in the XML. Now I have a different problem, though. When I test the sounds in-game, they get cut off before they finish playing.
If the sound is a faction soundtrack, then I cannot help you; your problem is most definitely in the soundtrack file itself. If it is a unit's sound, this can be solved in the XML of that unit. Also, I think the sound will be cut off if you select the unit before the previous sound is done playing.
For the sound quality:
If you are using audacity you can try to adjust the microphone sensitivity ( slider on top ) . This can help in some cases. I have 2 computers where this slider doesn't has any effect, but on another its impossible to record without adjusting it.
For the size:
Use for example vlc from videolan.org to see the what the original glest wav format is. Its something like "stream and media info" in vlc ( I only have the german version ).
Most of the original glest sounds are mono so they reduce their size by half. I use big stereo files ignoring these things 
.
Original glest sounds are something like:
Codec: araw
Channels: 1
22050 Hz
Bits per sample 16
bitrate: 352 kb/s
I use for example:
araw
2 channels
44100 Hz
Bits per sample 8
bitrate: 705 kb/s
Thank you Titi, this is valuable info. One time I tried to fix the menu and into music (ever notice that it kind of skips a beat in the beginning?), but the sound was always messed up and I ended up messing around with the bit rate for a long time.
Also, I try to record not with my computer, but with a separate sound recording device that I can then hook up to my computer to edit with Audacity.
A valuable tip with making sounds for Glest: use multiple tracks then mix them together. Then, by modifying one track's speed or pitch or some of the special effects that Audacity has, you can make the sounds original from the same recording!
My opinion on sounds with Glest is that the faction music is absolutely fantastic, but the menu music could use a whole lot of work. It would be nice to get some pro that maybe joins the forums someday to remake the music. The reason I do not like the menu music is that it is way too repetitive and not nearly long enough. It is also kind of an "in-your-face" type thing whereas many games (I think that the wisdom of the masses applies here) are more subtle but still unsettling. The difference between Glest and most other fantasy/strategy games is then that Glest is just simply louder and more obnoxious than it has to be.
