You cannot directly upload audio files as such, but I have managed to work around this. Here are steps to get audio files on tree using Windows XP:-
1. Disconnect any webcam you have attached (so webcam microphone doesn't conflict).
2. Start, Control Panel, Sounds and Audio Devices, Audio tab, Volume button (under Recording section), Options, Properties, tick all available devices, OK, select one of the following Mono Out, Line Out or Wave Out (names you have may not match or if unlucky you may have no suitable devices). Adjust volume slider for selected item to about 75% - may need further adjustment for best result.
3. Open audio file to be recorded and pause it.
4. Ancestry.com - press Record audio etc till recording starts. Note: Do not use PC for anything else while recording for best result. Broadband upload speed may affect quality.
5. Play audio file you paused. Note recording max time is 12 minutes.
6. Stop recording when finished. Save. Hopefully you now have good recording on tree.
I gather Ancestry.com reduces audio recordings to a low bitrate to reduce file size. By using a lower bitrate version of your audio file you may get a better result although that process is a bit technical.
Hope this helps.