I don't believe it's possible to do this within the Source Monitor. One option is as follows:
- add the clip to a sequence
- double-click the clip in the Timeline, which will load it in the Source Monitor.
- In the Source Mon's flyout menu, enable "Gang Source and Program"
- Set Source Mon to display the Waveform
- scrub & play from either the Source Mon or Program Mon.
I don't know whether this will work for your purpose of cutting out unwanted audio...