What boombim describes as 'distorted and dirty signal' could be jitter. Latency plays a role there, as bsduck already pointed out. But audio playing a few times quicker probably has to do with different resolutions. I don't know the Motu M2, looking at their website I see it's USB 2.0 audio class compliant. On Mac/iOS it doesn't need driver software, then it should work with FBSD too.
Finding the right settings is a bit of a hassle.
ffprobe -hide_banner -show_streams -f oss -i /dev/dsp3
is better for incoming streams (ffprobe is part of ffmpeg).
Apparently sound is not that important for us
To me it is. I'm running an external dac/adc in bitperfect mode using snd_hda, oss, audacity and audacious. Finding out how to use it took me some time when I started with it. But it was certainly worth the trouble now that it is running fine.