Do you have individual files or do you have folders on that USB drive? If you have folders, try loading individual tracks instead. Folder seem to confuse digital audio players as they are looking for specific codecs. Save your folders elsewhere and try individual tracks.
MTP is only a file transfer protocol and not an audio codec such a .mp3, .wav, etc... I am not sure why Suzuki would include that instruction as MTP does not read nor play audio codecs. Rather, it transfers them between devices.
One other potential solution is to format a new USB drive as a MTP drive so that the files transfer from the USB to your audio system.