PACX has the ability to play custom audio files. These files must be 16-bit or 24-bit WAVE audio. Other formats, such as MP3, AAC, or 32-bit WAVE are not supported.
A simple online tool can be used to convert your audio appropriately: https://audio.online-convert.com/convert-to-wav
You will have the option to change the audio bitrate as appropriate for use in PACX.