This is actually a trick to make it seem as if the wall is playing a sound. It's really a hidden switch which is causing the sound.
In this example, we'll be creating a telephone which plays the
dial-tone when operated. Place a switch over the telephone and tag it [ActivationSound,Channel]
. The dial-tone sound is #174. If you don't want the switch to activate anything, give Channel a unique value.
Angle the switch towards the wall. This should make it invisible (if not, just turn it back around, press 1
on it, then make it face the wall again). Now when the phone is operated, it will play the dial-tone sound.