For this issue with the audio, is necessary to determine if the issue is with the Slingbox itself or the signal that the unit is receiving through the internal tuner. We need to get more details about the tests that you have performed; did you connect the coaxial to the Slingbox from the wall or using the Satellite receiver? If you have connected the unit to the receiver using the internal tuner, you should be getting audio normally in all the channels, if you don't get that in some channels, disconnect the Slingbox from the receiver and using the same coaxial cable, connect a regular TV to see if the audio still works.
Finally, if the issue persists, I recommend to reset the Slingbox to factory settings and then run the Setup at Slingbox.com. Please come back if this doesn't work and also check the type of channels where you have this issue, like standard or HD channels.