Macbook Pro no longer outputting sound via HDMI

All

I have a 2009 Macbook Pro ally 13" running 10.6.8.

I have used it a few times with the mini display port adaptor and a HDMI lead to connect to my Panasonic TV and it carries the sound and picture, as an additional screen rather than mirrored.

All of a sudden a little while ago it stopped taking the sound, picture is fine but Sound isnt. My other half has the same laptop and when mine didnt work hers did and vice versa its very strange.

Anyway i have run through other peoples suggestions of changing settings but when i check mine no output settings in Audio MIDI are available to be changed.

I have run this through the AV reciever instead incase it was the TV but stil no luck, i know the cable works as it has been working before and carries the picture fine.

any suggestions as to what may be the problem here please?

Thanks