Exporting from Premiere Pro CS3

Good morning. I am trying to create autoplay files for my BrightSign interactive sign controller using Premiere Pro CS3 (I'm not sure which model number). The problem with this is that Premiere does not export the audio in a MPEG-2 transport stream as MPEG Layer 3 (mp3). It will only export the audio as MPEG Layers 1 and 2, PCM or Dolby Digital. Trying to use one of these other formats gives me a .mpg video file that the unit will not play and show an error on. According to the "HD Compact Users Guide," the MPEG-2 video files must have mp3 audio. Anybody know a workaround to this issue? I'm currently using a third-party file converter which makes the video files look sub-par, but maybe somebody can suggest a better export/conversion technique. I also read the "Optimizing BrightSign Video Quality" document and didn't find a solution there. Thanx.