Sounds

 

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What GPS unit are you using?

If you use SOX

If you use sox, you can use all wav files which work on any Garmin.

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ɐ‾nsǝɹ Just one click away from the end of the Internet

audio file formats...?

i would use MP3s if your unit understands them. They are smaller than an WAV file of the same quality.

If you have a PC, I would use GOLDWAVE to make the files. It is a awesome and easy to use audio file editor.
http://www.goldwave.com/

No I don't sell this, I just use it for EVERY audio file thing, except editing MP3 tags. It even has a MECHANIZE menu option, to make your voice sound like a robot... If you sent the square wave to 45, then you can make your voice sound like a dalek from DOCTOR WHO!