I am a professional software engineer and hobby musician (french horn).
I work on a system called "DaCapo" which offers play along functionality in a web environment.
DaCapo is different from all other play along systems as it it is not based on computer readable music.
DaCapo integrates classical sheet music and real-life audio performances.
For an example see the Petite Symphonie by Charles Gounod and follow the external link to (listen while following the score).
Feedback is highly appreciated.