There is an issue with OSMC audio output when using an asoundrc or asound.conf file.
First time when you boot it will list and use correctly the USB soundcard in system >> sound, but once you use the device configured in asoudrc with another app (like mpd) the soundcard disappears from soud listing in KODI and switches to HDMI. I am using alsaequal.
Is this a know bug?
For anyone interested, i’ve found a fix using dmix.
For example, if you want to use alsaequal plugin, go for dmix: