When I try and render my comp as .avi format, when I try and play it back with Windows Media Player, there is no sound! Can anyone help?
Nobody can tell you anything without proper system info or other technical details like version of AE, exact comp and render settings, source files used, system audio settings and so on. Just saying that there is no sound is not useful.
This is a confirmed bug in the bug fix. A staff member on another thread confirmed it a few days ago. You can try different configurations of format & codecs and send your findings to adobe. Or you can revert back to 2014 or earlier to altogether avoid the many issues with audio in AE 2015