and others I need to turn the volume way up and that is when I hear the buzz much more.
This description make me, too, suspect that there may be a conncection issue. All connections between audio devices have some level of electrical noise (buzz). I originally missed the point about you having to turn the volume up. Knowing that other equipment works fine tends to point to something other than wiring (but may be a connection issue).
The fact that you are having to turn your volume up until you hear that noise indicates a problem that the signal level (audio signal) is not high enough compare to the noise, known as signal-to-noise ratio. I would expect a proper signal to be able to make it over 70 feet. The problem with wires this long is not loss of signal, but the introduction of electronic noise. Connections can surely be one potential problem here and can cause signal drop where none should exist. Bad connections can also cause additional noise. The fact that your setup works with some equipment and not others would make me consider connections as a likely problem. I would definitely check these, again.
I am also curious about the existing cables. Did your installer put these together himself? Is it possible that there is a connection mid-way, joining two or more shorter cables to make the 70' run? I would make sure that each of the five wires between rooms is a single run, without joints or connections between.
From what in the theater room are you sending the audio/video signal. Is there a way to control volume level from this device? Is it possible that this level is set too low? If you are willing to spend a little money to experiment around, you may consider running to radio shack or fry's and purchasing another long cable (2 wires is enough) to test your connections. Remove the in-house connections from your two audio outputs and temporarily run your new cable to from there to the tv audio inputs. If this solves your problems, then this points to a problem with your cables.
The suggestions to use shielded cable, direct box, balanced connections...all would necessitate replacement of wires. You would not be using your existing wiring. I would consider this to be pretty drastic and would not pursue it until we make sure that simpler solutions don't solve your problem.