The sampling rate can be anything you want.
OctaMED, like other trackers, works on the principal that each note corresponds to a playback frequency (well actually it's a playback period but it amounts to the same thing). Generally, (for normal samples in 4 channel mode), F-3 corresponds roughly to 22kHz. Hence, regardless of what the sample frequency was originally, if you play it at F-3 it will play back the sample data at roughly 22kHz.
If your sample is 22kHz, it should sound about normal at that note.
In mix mode, or if you use an 'ext sample', the octave range is larger, typically F-5 then corresponds to 22kHz playback rate.
-edit-
If you want to make high quality music, I reccomend several things
1) Use mix mode for preview but when done, render your track to disk with smoothing enabled.
2) Use 16-bit 44kHz samples for percussion and any musical sound effect samples and play them as close to their target frequency as you can (unless really changing the sound of course). Note that samples played low will have the aliasing sounds you are famililar with but with smoothing enabled they'll be lost.
3) If you really want to get good results, render your main bits to disk as seperate audio tracks and then post mix them in a good package. You can apply effects etc to seperate parts this way that octamed itself cannot reproduce well. By using seperate tracks you can preserve that grungy aliased sound for bits you need and use proper interpolated high quality sound for parts you don't.
4) Try using the synth sound editor for creating 808-like bass sounds. You'd be seriously surprised how good it can sound having done all the above ;-)
