I assumed .ogg files were compressed to save ram, but the compression could be causing a load problem.
Sounds plausible. If that is the case the quickest & most significant gain would be to convert frequently played, small, sounds from .ogg to .wav format - weapon/ability effects and engine loops spring to mind, especially as several play at the same time.
Music maybe a factor but that tends to be large and only one tune plays at a time, so conversion may not save much CPU load (but could use more RAM).
Converting dialogue sounds (unit orders and GameEventSounds) probably would have the least benefit as thy are heard comparatively infrequently.
If anyone uses Cygwin then the 'sox' command can be used to convert from one format to another.
Eg: Convert SOA2's Borg mantra
$ sox BORG_Effect_WeAreTheBorg.ogg BORG_Effect_WeAreTheBorg.wav
This does show that .ogg is compressed when compared to .wav - 111kB for .ogg versus 1.4Mb for .wav.
However, unless .ogg's can be sent directly to the OS's audio subsystem (like some texture formats can be loaded directly by video drivers) this difference may not reflect an increase in RAM usage as Sins would have to uncompress the .ogg into a larger memory buffer (to store the compatible format) before playing. So converting from .ogg to .wav may not be that bad in terms of memory usage.
After all .oggs may have been chosen to reduce the size of the stuff written to DVD (back in the old days) or to reduce the size of the download via Steam (or Impulse, again back in the old days)