OEM is Original Equipment Manufacturer. The company the built the chassis. Ford, Chevy etc. There would be a fuse box that came with the bare chassis. that fuse box wold have the normal fuses for lights, Dash A/C, wipers stc.
Coach fuses would be a fuse box installed by the coach builder. That's where anything installed by the coach builder would be in that fuse box. Awning, steps, leveling jacks etc. There may be 2
pr 3 of these.
Can't help you with the speaker problem but I'm sure there is a way to access those fuses.