There are a total of 17 musical instruments in Metro Last Light. Finding all of them will earn you the Musician trophy or achievement. There are only 4 types of instruments: Guitar, Balalaika, Accordion, Piano. You have to find and play all 17 though. Playing one instrument of each type is not enough to unlock the trophy. You have to find all of them in one playthrough. Doing this via chapter select won't work.
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via N4G: pc news feed http://n4g.com/news/1257578/metro-last-light-all-musical-instrument-locations