Hi babymum, have you tried using the Audio Diagnostic Tool?
In case you haven't please give it a try -- the tool will automatically apply the most common fixes to audio issues, and after each "round" you may go back and test LR to check if the audio plays better. If not, just switch back to the Audio Diagnostic Tool to try another fix.
For details on how to use the Audio Diagnostic Tool, see:
http://support.brillkids.com/entries/20433747-how-to-use-the-audio-diagnostic-toolIf you reached the last step of the tool and the audio problem remains unresolved, do write to us at support[at]brillkids[dot]com and attach your Diagnostic Report so we can troubleshoot further.