How to Use Real Musical Instruments with GarageBand on Your iPad. In here you’re able to switch Metronome to off, but leave Count-In on. Singers can sing along to tracks created in. Sep 30, 2011 I haven't use GarageBand in a while, but I'm pretty sure you can't change the volume of the metronome. Forgive me for stating the obvious but you could try to keep the level on your pre-amp/audio card clean but lower the guitar's volume in garageband while recording and then just increase the overall volume?
GarageBand User Guide for iPad

If you have an external keyboard connected to your iPad, you can use keyboard shortcuts in Tracks view to play, navigate, and edit your GarageBand song.
Can You Mute Metronome On Garageband On Ipad 2

Can You Mute Metronome On Garageband On Ipad Youtube
Action | Shortcut |
|---|---|
Start or stop playback | Space bar |
Go to the beginning | Return |
Move forward | Period (.) |
Move back | Comma (,) |
Create a new track | Option-Command-N |
Duplicate the selected track | Command-D |
Delete the selected track | Command-Delete |
Select previous track | Up Arrow |
Select next track | Down Arrow |
Solo the selected track | S |
Mute the selected track | M |
Undo | Command-Z |
Redo | Shift-Command-Z |
Select all regions or cells | Command-A |
Delete the selected regions | Delete |
Select the previous region on the selected track | Left Arrow |
Select the next region on the selected track | Right Arrow |
Add previous region to selection | Shift-Left Arrow |
Add next region to selection | Shift-Right Arrow |
Split selected region or event at the playhead | Command-T |
Join selected regions or notes | Command-J |
Cut | Command-X |
Copy | Command-C |
Paste | Command-V |
Loop the selected region | L |
Start recording | R |
Turn the metronome on/off | K |
Turn count-in on/off | Shift-K |
Show/Hide Remix FX | Option-R |
Show/Hide Automation | A |
Open Loop Browser | O |
Instrument Browser - Previous Instrument | Left Arrow |
Instrument Browser - Next Instrument | Right Arrow |
Show/Hide coaching tips | H |
Show Help | Option-H |