I have the oem surround sound in my bighorn, and it seems to be a quality system, so I see no need to replace it...
my trailer didnt come with a music stereo, so when I finally got around to installing one, I had to build a speaker switch so that the surround could either play from the TV/DVD system, or switched over to play from the stereo system..... by default, after installing the switch so that the speakers could be individually switched on or off, and from tv to stereo, it also gave me a good access point to run outside speraker wires from..... I used RCA jacks at the termination end of the wires, and just plug my outdoor speakers in when I want to use them..... the speakers run off the device they are connected to, thru the speaker switch, so if we are on 120vac power to watch tv, OR, if we are on 12vdc listening to music, the speakers will always work...
I used (6) DPDT on-off-on toggle switches and a project box to make the switching apparatus.
some speakers have a cover that can be removed, or the speaker itself may be able to be pulled form the housing, and if this can be done, the housings can be drilled so they can be mounted to a surface without the need for the ugly mountings that some come with... the speakers in my bighorn is mounted in this way.... pull the speaker out and see the screws holding the housing to the ceiling....