How does music affect heartbeat, if at all?
On a different (perhaps related) note, why do stoned listeners like to hear high BPM music (trance…)? Is there some kind of “coordination”/“resonance” between the increased heart rate (mine goes upto 140) and the tempo?
I thought stoned (on pot) listeners preferred to listen to slow languorous music: reggae, hip hop (particularly g-funk), jam bands etc.
Or do you mean stoned as in clubbing associated drugs?
I think it depends entirely on the drug and the person.
Anyone know about the first question?
Hmm, I guess it could affect it indirectly. Many times when a song or piece of music comes up it triggers a memory (of, for example, a place I was in when I first listened to it) which would then set my heart racing temporarily. 
I just saw a poster on this at the state fair’s science project competition. Some kid measured the effects of fast rock and slow classical music on resting heart rate. The fast music raised heart rate by ~10 beats per minute, while the soothing tunes lowered it by ~5bpm.