You can assign sounds to rhythms in PolyNome on the Edit Pattern page. Simply tap below the notation to bring up options for assigning sounds.
You can change the assigned sounds, and those for the Metronome, in the Sound Picker.
To bring up the Sound Picker from the Edit Pattern page: tap and hold one of the Sound Buttons (R, L, K, H)
To bring up the Sound Picker from the main page: Tap in Seq1 and choose Sound Picker. OR, in the Mixer, tap on a speaker button above the faders.
To bring up the Sound Picker for a Playlist: Tap the Playlist name, then "Change Sounds For Playlist"
Watch the video to learn more: