Okay, this is REALLY weird.
I dug out my old project (from three years ago) and began working on it again (with AGS 2.72), when I wanted to implement my first sound effect. There was already MIDI music, but this was a WAV sound. So I tried to import it, dunno if that worked, but what happened is that the game window didn't update while the window was located too far up on my screen. Too far means in this case, above ca. the middle line. Taking the sound effect off the project didn't solve the problem.
I took this as an opportunity to install the newest AGS version. Everything went back to normal. Then I converted the sound file to MP3 and imported it again. Same thing happened as before! The music was playing, but the window showed nothing until I moved it down, and unless moved far enough down, nothing moved in it.
Now, as if to mock me, it went back to normal a few minutes ago... not sure what I did to make this happen, I didn't touch the sound file or import section. But now that I typed all this, I think I will post it anyway. You never know...
Strange, it might be related to your video card drivers somehow. If you switch from DX5 to D3D or vice-versa, does that help?
Sorry, I can't check that because it didn't happen again. But I didn't use any sound since then, so it might happen again, and if it does I will try that.