Q: Mid 2010 Macbook Pro - sound output to HDMI?

Check that your model supports audio over mini-displayport, not all of them do:

http://support.apple.com/kb/HT4241

If you're sure it does, then check you have the correct output selected.  Hold down the option key while you click the speaker icon in the menu bar.  You can select the output device there.

If still not working, click the Apple icon on the left of the menu bar and select 'Software Update' and make sure your system is up to date.

If still not working try resetting the PRAM first, and if still not working reset the SMC:

http://support.apple.com/kb/ht1379

http://support.apple.com/kb/HT3964