There is not dedicated reset command for a Track.
When you create a "Guitar" Audio Track, it will have the Amp Designer and Pedalboard plugin already loaded. When you create a "Mic" Audio Track, it seems to have that basic Patch settings loaded. That is as close of a default reset you can get and I think that is what you wanted.
Here is what you do:
- Create a new "Mic" Audio Track
- Open the Library Window and click "Save" in the lower right corner of that window. This lets you create your own custom Patch
- A Dialog Window pops up, where you enter a name for your Patch , i.e. "Default Patch"
- Now, select the Track you want to "reset"
- In the Library Window a new Category "User Patches" has been created where the "Default Patch" has been saved to. Click on the "Default Patch"
- This will load the Patch onto the selected Track
- That's it

You can more information about Patches, User Patches and the Library in my new book: "GarageBand X - How it Works"
You can use the custom Patches not only for this one but in general to store your favorite SOunds you created on your Track. It also stores any AU plugins that you have used on a Track.
Hope that helps
Edgar Rothermich
http://DingDingMusic.com/Manuals/
'I may receive some form of compensation, financial or otherwise, from my recommendation or link.'