Handle source content switching from 5.1 to 2.0 and back to 5.1

[cannonbeach]

This seems to be happening during some commercial inserts- the audio is changing from 5.1 during the primary program to a splice of 2.0 during the commercial and then back to 5.1 during the primary program again. Go figure!

[cannonbeach]

This bug causes the audio to sound bad during the commercial. It was found without using the "--stereo" downmix command so I am retesting the same source with the "--stereo" setting enabled.

[cannonbeach]

This appears to be working now, but the commercials are a bit quieter than the normal programming.