Oxford University Interview?

Don't worry, you'll only have to play an instrument if you're applying for something like a music scholarship! In the interview they very probably won't mention your extracurricular activities, since the interviewers need to use as much time as possible asking subject-based questions.

It is also unlikely that they will bring up any extracurricular activities unless they are strongly related to your subject, in which case they may ask you what you learned or what you gained; for example, I applied for Greek/Latin and in my interview I was asked about the Greek mythology club I ran for younger kids in my school. But as I say, it's most likely that your subject is the only thing that will come up!

Hope that helps!