We need to select a format that can be output by the mixer, preferably
the exact format output by the mixer. Keeping the input audio format
breaks for non-linear input formats when S/PDIF output is disabled:
the mixer outputs FL32 that cannot be converted to MPGA, AC-3, etc.
(cherry picked from commit 1640b6840ac8d6f08b527950b6e30b9035bbe6fe)