Click the name of the GarageBand track you want to remove a cough from. If you wish to take a closer look at this track, double-click it to open it in the “Track Editor.”
Click the playhead cursor, which is found at the top of the GarageBand screen. The playhead cursor controls where playback begins in GarageBand.
Drag the playhead cursor so that it is just before the cough you wish to remove.
Press “Command” and press “T” to splice this track where the playhead cursor is.
Place your mouse over the splice and it will turn into the “Trim” tool. The "Trim" tool looks like a vertical line with an arrow pointing to the left or the right. The direction the arrow points determines which region the "Trim" tool will affect.
Click your mouse when the “Trim” tool appears and drag it until the section of the track containing the cough can no longer be seen. This will trim away the audio region up to this point, removing the cough from the track.