The MM chip (VS1053) wants to be fed data constantly so that it keeps the music playing. You can take small pauses to do stuff in between playing, and for real-time sensors like IR receivers, you'll have to not-use interrupt based playing and instead pause to read. 

No there is not.

No. But you can adjust the volume up in +6 dB increments with the +6dB and +12dB jumpers (but be very careful doing so).