How to 'difference' one vector from another?

I'm trying to create a mute music button:

And I'm doing it by drawing 2 vector lines over a music note vector. I've tried selecting them every which way but whenever I try to take the path → difference nothing happens. Ideally I'd like to subtract the big like (white in the above image) and add the small line.

How would I do this?

Solution

Got it. You have to be in the edit paths mode:

You may also need to convert the object into a path. Path → Object to Path