"Left edge of synth: right arrow, right edge: left arrow" may prove confusing to mouse (or touch) users, who will have to move the mouse cursor (or their finger) in the opposite direction that the single arrow cursor is pointing in order to move in that direction into the synth.
It makes sense for keyboard users, though.
The single image on each border of the synth can go by pretty quickly.
Were you thinking of showing the one directional cursor arrow only on the very last image on both ends of the synth or perhaps the last few images on each end?
If the starting point is at one of the ends, most people will have the synth starting to play before they can begin manually navigating. This means that they would quickly have moved away from the one image at the end where the one directional cursor is shown and by the time they see their cursor, they'll be into the two directional images.
That's where your request for some sort of progress bar enters, though.