22a Know-all - 'kno(b) + wall(y)' (stupid people, both contracted)
23d Owns up - 'own' + 's(tatement)' (primarily) + 'up' (in court)
9a,13d Soundtrack
1d US-ER
4d Assenter - 'ass' (American "behind") + 'enter' (record)
15a Yes - 'So up' (well excited) + 'con' (do; trick)