The most likely reason for this is that Windows Media Player 9 and 10 both have a built-in behavior setting where they analyze the file to transfer. If they think the audio quality can be improved by converting the MP3 file to WMA, they do so before transferring the file. For more information on this, please see this Knowledge Base article.

Basically Windows Media Player is trying to convert the files to WMA format which Basic does not support. The above Knowledge Base article contains the solution.

If you are having trouble connecting to Windows Media Player with Pocket Tunes, please see these articles:

Windows Media Player does not list my device in the drop down of the "Sync" window(or "Copy" window in WMP9) see this Knowledge Base article.

You can use Legacy communications, for more information, see this Knowledge Base article.