OVA #6: Name of song at 0:18? So I heard this song many times but I have no clue what its name is. It's in OVA 5 (when Sonic was training) and OVA 6 (S class hero murder, at 0:18). It's the trumpet soundtrack.

I really want to find the music sheet for that, because I play trumpet. 
Thank you so muuuuch  (´▽`)/
This is track #42 "Yuuhi ni Mukatte" from the One Punch Man soundtrack. Here is a 20-second extract from the beginning of the piece: https://clyp.it/m3qa2cee.

I don't know where you'd find sheet music for it. It is rare for anime soundtracks to have accompanying sheet music (though it does happen, e.g. Symphogear, K-ON!). Your best bet is likely to hope that an enterprising fan has transcribed the piece.