Tap on a metronome and choose "Sound Picker". This will allow you to choose sounds for the metronome.
If no stickings are entered for the pattern then you'll be able to select a single sound.
If you've entered stickings on the Edit Pattern page (by tapping under the notes and using the R,L,K,H buttons) then you'll be able to choose sounds for each of the letters used.
You can also choose sounds for the letters while in the Edit Pattern page entering sounds. Tap and hold one of the R,L,K,H buttons to bring up the sound picker.