Well... I don't like recommending this method all over the forums, since it is advanced, but as I haven't found a simpler way:

Install Easy Debian Chroot; this lets you have a Debian system on your SD card and run programs from it inside Diablo.

Use Debian's gnome-alsamixer, alsamixer, or any other ALSA mixer program (needn't be from Debian, but AFAIK one doesn't exist for Maemo) to adjust the DMA control up maybe 4 notches.

Easier options (depending on what you're trying to play, and how you're playing it) might include losslessly amping your mp3s with mp3gain or configuring mplayer with softvolume, but the DMA tweak works for all sounds.