Hi
I just joined a moment ago so I could post this.
I had the same problem - Since updating my new Win7 computer to Win 8 2 days ago, I noticed I was getting emails in my email client (Thunderbird), but not hearing the soundbite I'd assigned to it just a few days ago (Gene Wilder's cry of "It's Alive!" from Young Frankenstein), and Googled it for a solution - nothing concrete in any of the first 5 or 6 pages of links, so I tried a few things myself. Found a fix that worked for me, and wanted to share.
I did check and verify that both Thunderbird's options and Win8's Sound applet in the control board were set to use te sound bit, but I noticed that in the sounds tab form the control panel, only about 30% of the installed files would play from there, so that got me thinking that some WAV files might be in a format that Win8 couldn't play, and that was causing the problem; Win8 was trying to play the file, but not interpreting it as sound.
I downloaded and installed Win8 codecs pack from MajorGeeks site (being careful not to install the stuff that tries to piggyback into your system with the install, a tool bar, popup ad/coupon program, home page change, etc). Then I downloaded and installed the Win8 drivers for my audio chip from the HP site. Rebooted, and all seems fine now.
Hope this helps someone else.