Creating MP3 or WAV files

Not sure what you are really trying to do. People here often use spreadsheets like Excel as a crude editor for poi .csv files (which are actually text files that can be edited in almost any editor). The reference to Excel makes one think you wish to create a WAV for each entry in a POI. But if that is the case then you should know that POI loader only associates a WAV (or MP3 on models that supported them) with the entire file, not different WAVs for each entry.

I generally don't care for the quality of text-to-speech programs. If that is really what you want then just find one that you like. I don't know of any that input Excel format files, but since .csv files are really text files you should be able to edit your list and have it ready for your text-to-speech program with any simple text editor. Or let me suggest that you may want to look at Audacity, a free and very nice sound recorder and editor. With Audacity you could just read off your list and record it to a sound file. Then you could look at the waveform and cleanly edit just the parts that you want and export them into separate .wav files (or MP3s). Even if you use a text-to-speech converter you may wish to use Audacity to do your final cleanup, trimming dead space or extraneous noises and adjusting levels so that the final file is the same level as other audio on your GPS, as well as doing MP3 conversions if desired.

EDIT: On rereading I saw the mention of tourguides, so I better understand what you are doing. My above advice still holds, I think it is unlikely that you will find just what you want in a text-to-speech system that reads Excel files, end even more unlikely that you will find one that inputs the XML formatted files that tourguides need. But you should certainly be able to produce a simple text list and go from there.