How far away from the main panel is the listening room and the two bedrooms and bathroom? The only reasons for a subpanel is to limit the voltage drop over long distances, or to group together outlets in a common area so as not to overstuff the main panel (i.e. a garage woodworking shop with numerous 240V circuits for power tool motors).
To give you a perspective: a #12 branch line carrying a load of 10 amps can be run up to 75 feet to keep the voltage drop under 2.5%. That is a lot of amps. If you go with #10 wires, then you can go up to 125 feet for a 2.5% voltage drop.
Since you have a dedicated service drop for the second structure, I see no reason for a subpanel with only three audio lines, one GFI bathroom circuit and one or two AFI bedroom circuits. Unless it is quite a big house.
To give you a perspective: a #12 branch line carrying a load of 10 amps can be run up to 75 feet to keep the voltage drop under 2.5%. That is a lot of amps. If you go with #10 wires, then you can go up to 125 feet for a 2.5% voltage drop.
Since you have a dedicated service drop for the second structure, I see no reason for a subpanel with only three audio lines, one GFI bathroom circuit and one or two AFI bedroom circuits. Unless it is quite a big house.