One thought would be to double check the folder structure in the SD card, the mixer files you want to use need to be in a folder /etc/mixers in the sd card.
that is the SD card needs to have this structure
/etc/mixers/your_mixer_here.main.mix
this will be reported by the init script as:
INFO [init] Mixer: /fs/microsd/etc/mixers/your_mixer_here.main.mix on /dev/pwm_output0
at least this is the case in a nuttx based system