There's a wide range of music in Chan-wook Park's Stoker. A psychological thriller that mixes up possible and actual memory, the music also regularly hops across the diegesis. This is partly an effect that works in tandem with the rich and carefully prepared sound design, as much a character in this stylised film as any visual sleight of hand.
The music itself made its impresion on me in three ways. Firstly, we hear an opera, both on the radio and non-diegetically. Verdi's Il Trovatore is a somewhat classic opera plot in which familial passions and a crucial obfuscation of the facts leads to murder. This is (not to spoil it) not too far from the main themes of Stoker, though Chan-wook's film is rather more surreal and pathologically curdled than the more straightforward red-mistiness of the opera. This is the extract from the film, Viorica Cortez singing Stride la Vampa... an aria of vengeance for a murdered parent:
More interesting is the music played at the piano. This/ese duet scenes use the music of Philip Glass, apparently specially written pieces for the film. Rather like the similar scenes in Jane Campion's The Piano of twenty years ago this Gothic-styled, eroticised set-piece involves fairly simple music that builds in intensity (if not complexity). Here's Michael Nyman's original, of 1993 - compare it with Glass' Stoker piano work.
Finally - and the bit I haven't quite thought through (!) - is the use of whistling. I think that people use whistling as a displacement activity, to take their mind of tedium (usually) and I suspect that the use of whistling indicates subplot, whether actual or psychological.