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 press 1 on it). Now when the phone is operated, it will play the dial-tone sound.