Is repentance for the specific act one wishes to repent from, or for all sins? And is repentance from listening to music accepted if the person returns to listening to it and repents again?

May Allah reward you for your advice. The written greeting (salam) must be verbally responded to upon receipt. If someone reads the greeting and replies to it, that suffices, and they are not required to reply in writing. Repentance from all sins must be immediate, and repentance from one sin without another is valid according to the majority of scholars. Returning to a sin does not invalidate the first repentance, and the servant will be held accountable for what they committed after repenting. If they return to sin and then repent again, their second repentance will be accepted with its conditions.