Google doesn’t allow uploading mp3 files as GV greetings (speculation on the forums is that they fear lawsuits due to copywrite violation) but there is a trick you can use.

You can use your iPhone’s recording app to record your greeting. It will be saved as an .m4a file, but if you rename it to .mp3, it will still play ok.

1. Queue your .mp3 file in your computer media player

2. Go into your Windows Sound settings (right click the speaker icon in your Task bar or go through Control Panel) and select Recording Devices

3. Change your Recording Device to Stereo Mix

Now that your Windows Recording Device is set to Stereo Mix, whatever plays from your media player will be channeled in place of your microphone.

Go to Google Voice Settings > Voicemail & Text > Voicemail Greeting

[NOTE: I didn’t find this part to be correct. I didn’t have a phone tied to the Gogole Voice account and it still worked. YMMV…]

You must have a software telephone configured on your computer desktop (and verified as a Google Voice forwarding telephone) that is able to answer a call from Google Voice.

Click on Record New (in Google Voice) and select your verified software telephone. Answer the phone when it rings

Google instructs you to record your message after the tone. When you hear the tone, play your media player with the .mp3 file that you queued in step 1, above

Your .mp3 file will go directly through “stereo mix” and record to your Google Voice voicemail greeting bypassing the telephone microphone. Your recording should be a lot cleaner than if you used a headset/microphone although it won’t be full .mp3 quality as if you physically uploaded it.

When you are finished, remember to go back to your Windows Sound settings and change Stereo Mix back to Microphone.