Sony has released an update to Media Go that solves this problem. Please make sure you have the latest version installed.
If you cannot update for some reason, you can use the following steps to convert MP4 files to a different file format during transfer, as an alternative solution:
- Right-click on the Device in Media Go and select Device Preferences from the context menu
- Select the Audio tab in the Device Preferences dialog
- Set the "Format" to MP3 or FLAC (if supported by your Walkman device)
- Make sure the "Only encode audio when required by the device" option is unselected