The condition returns that song is seekable, but setting position still doesn't work. Is this the limitation you're talking about or if it's seekable it should work.
What is the file format you're using? I've only tested setting position with wav files. As I recall if the music has stopped playing and you use set position you need to play it again to get it going.
Is there any difference between using the Music actions and Sound actions? Because sound actions are lacking the option to repeat sounds plus "Set volume" doesn't seem to work for sounds.
Music streams a file and Sounds load entirely into memory first. Sounds also are played using a feature of the audiere library where the sound is loaded only once and every time you play it another instance of the sound is played. An unfortunate side effect is the set volume action will only affect the most recently created instance of the sound.