I have a similar problem and the only way to temporarily fix it is to reboot the phone. I'm in Motorola Defy.
In my case, the first sound to disappeared is the attacking sound, then later the enemy calls sound. I tried the sound option in the game, but after disable and enabled it, the background music is gone too. Left only the environmental sound.

And the sound seems to disappeared and triggered when I traveled to another area. Hope this info help the technical team to fix the issue.