Well according to CDnow.com, this is the track listing on the soundtrack:
1. Mystic River Main Title
2. Abduction
3. Communion / Katies Absence
4. Jimmys Anguish
5. Meditation #1 Piano
6. Orchestral Variation #1 Of The Music From Mystic River
7. Escape From The Wolves
8. The Morgue
9. Brendans Love Of Katie
10. Daves Past
11. The Confrontation
12. The Resolution
13. A Full Heart
14. Meditation #3 Piano
15. Orchestral Variation #2 Of The Music From Mystic River
16. Theme From Mystic River
17. Cosmo / Kyle Eastwood
18. Black Emerald Blues / Kyle Eastwood
I searched for 'Mystic River' on SS and some guy had all the MP3's (they were named 'Clint Eastwood Mystic River xx.mp3' with the x being the track number)
So I dunno, just try searching on there and download the track that you think the song is 