For a lark I wanted to see if a BPi would do as a softsynth.
I started with the very nice 2.9 Jessie image* and 4.x kernel, but noticed only then that there's no sound, so I installed the 3.4 packages. After booting, ALSA seems to be working fine wrt line sound output and mixer. I could also play MIDI files with fluidsynth fine and hear clear audio on headphones. Connecting a USB MIDI keyboard also gets detected and its ports show up.
Now, fluidsynth as a server won't start and neither does jackd. Both seem to complain about missing /dev/snd/seq. So I'm wondering if there's a kernel module or option missing or if it's an artifact of there being no actual MIDI synth chip available. But it looks like neither fluidsynth nor jack can register themselves for routing MIDI at all. There might of course be some unrelated problem that I haven't noticed.
The device is not in reach now so I don't have exact output, but I thought I'd throw this topic up for a start.
* Thank you for the effort put into these! It's great to have ongoing development on these boards. Firstboot on 2.9 didn't work unless I resized the partition first to make some free space. I imagine this is probably fixed in a later image as the brokenness note on the page is gone.