No audio from .gsm sound file in Quicktime

We have a .GSM file which prior to a few days ago use to play audio quite happily within Quicktime 7.7.6 on Windows 7 x64bit Workstation, however this has now ceased on this particular workstation.

When you open the .GSM file, it opens correctly suggesting the file is not corrupt (no warnings or errors), and the equalizer is active and shows that the audio should be playing however no audio is heard through the headphones. Looking at Windows Volume Control it also suggests that audio is present and the sound levels are changing suggesting there should be an output.

Again I re-iterate that if I open for example .mp3 or .wav file these will play through both Quicktime and Windows Media Player successfully the only issue seems to be with .gsm files.

We have replaced the workstation the problem seems to happen on this workstation too.

- Changed Headphones and used both Front and Rear Headphone Ports on the Workstation.

- Re-installed and then also rolled back audio drivers

- Verified that all other sound formats will play within Quicktime, Windows Media Player and VLC Player with no problem.

- Re-created the users profile in Windows.

- Ran DirectX 11 re-installs and used the inbuilt DXDiag to identify if there were any issues with the sound card or DirectX.

- No Group Policy appears to affect the operation of Quicktime.

- Re-installed Quicktime

Any ideas?