It's currently not possible to delete from Inbox on iPad or iPhone, I'm afraid. I don't think it's been suggested as an idea before so feel free to create a new idea on the Ideas Exchange. (@Premify - sorry if it has been suggested before!)
Really? That hasn't been thought of by your product team or suggested by a customer at this point? That's odd. Inbox equates to email or MMS. The interaction model should follow up on that expectation.
I hope this feature is added soon. I would suspect most users have "that guy/gal" in their friend list that sends them music they would like to delete.