Is it permissible to revoke a vow to pray two rak'ahs whenever I hear music, especially given the presence of obsessive-compulsive disorder, and is expiation (kaffarah) obligatory for listening to music after the revocation?

It is not permissible to annul a vow after it has been made. Expiation is only required with absolute certainty that what the vow was contingent upon has occurred. However, if there is doubt, nothing is required. If one breaks a vow under the influence of obsessive thoughts (waswasa), no expiation is necessary, because a vow or covenant made under the influence of obsessive thoughts is not binding and has no validity.