1. Open the feed up in audio editing software like audacity.
2. If you can see a waveform but still can't hear it then you probably have to reconfigure your audio software.
3. If you can't see a waveform then there may have been no transmission at that time or the feed was down.
4. We could help you better if you tell us which archive it is; a) the actual file name b) the date and time of the feed c) the feed name - i.e. KLAX d) the actual local time that you wanted to listen to.
It could very well be that you haven't converted whatever time you are looking for to UTC time and you've actually downloaded a file from the early morning, a period when there isn't any activity, when you thought you were getting late morning or whatever.
Without the above information, there's not much else I can say.
Thank you for flying QANTAS - 'Quite A Nice Transmission, Any Sound?'
