The basic problem has already been addressed: the voice files are now too large. That's because the size of the firmware had grown since the release version (new features). Both - the firmware and the voice file have to fit in the 2MB of RAM the Archoses have.
*...and I think it's about 1.2 megs on archoses
It varies a lot and it's not a hard limit - on your OndioSP it should be more than this. My backlight modded OndioFM has about 1.3 MB of available buffer even though the FM and backlight related code adds a lot to the firmware size - compared to the SP.
it also depends a little on settings like max files in dir and max entries in playlist, try to keep these as low as possible.
Yes, you can influence it a bit with settings. Other than the already mentioned ones, there are for example the scrobbler log (which doesn't even work correctly) and cuesheet support that take away a lot of available buffer size too, in case you activate(d) them.
*) This is not true when you run rockbox from ROM (if you use the so called rombox) - the OndioSP is one of two targets where this is still possible with the recent versions. (Excuse me if I'm wrong in assuming that you don't).