I'm shocked that there's no keyboard shortcut for rewind / fast forward a song, on a great app like this one. There might be some strange reason why they decided not to implement (which shouldn't be a big deal at all) or activate it.
Not just on windows phone, also Win7 and Win 8. a tap on the arrow left key for 3 seconds or settable seconds would be great. Also, Ctrl+left arrow for 10 or 20 seconds. This is normal functionality seen in popular players like Gom and Pot player
If it's not possible to accommodate new buttons to this request you could use the previous/next buttons and use the hold for fast rewind/forward. Everyone would love it. If someone doesn't like it there could be a switch off config, or a holding time activation, so someone could set 10s to start the fast rewind/forward. It could be done even in the lockscreen gadget that I use so much =D (I set a list and listen to it all day, using my phone locked most of the time). It could still be used to jump to the previous/next song when someone clicks it, and fast rewind/forward if someone holds it, just like old portable cd players 😛