well when using ogg vorbis files i beleive they contain compression, or something like that but they DO require MORE processor power to play, so i wouldnt depend on underclocking your axim to 100mhz to save power, ide test one out on your highest bitrate of music such as 190, i do think 190 is overdoing it i beleive a normal cd is 128, unless you are refering to variable?
i would honestly recomend mp3 due to its popularity, if you dont want to buy a mp3 ripper then there is a free one, is has adds but you can pay to get rid of em, it has advanced features such as variable bitrate which can save space.
For example... if there is silence in your music track then it will lower the current bitrate to 32kbps compared to the normal 128 or 96kbps.
Heres a link
http://www.pcworld.com/downloads/fil...d,16726,00.asp
I beleive it also convert vorbis files, and controls bitrates to.
hope i have been some help
oh and ajw very helpfull first post and welcome to the forum!, i hope all your future axim problems just fade away