i am having toruble with my mustang?

[cobrajet]

The brakes have nothing to do with any of the rest of your problems. They don't depend on anything electric to work for safety reasons. If they don't work it is either a leak in the rear hydraulic system, a mechanical failure/worn out parts inside the drums, the proportioning valve is no good, or the master cylinder is no good. If the brake light is on you should take it to a mechanic IMMEDIATELY. No rear brakes will get you into an accident very easily. Applying the brakes when going around a turn or in wet conditions will make the vehicle spin out.
The sound system problem can be a burnt out amplifier in the radio or an external amp, a blown out channel inside the radio, a blown fuse, a bad connection somewhere, or blown speakers.
The high beams not working are either because you have a bad headlight switch or because both bulbs have the high beam filaments burnt out.