This issue is a proper rephrase of #2173, since the problem isn't clearly stated there.
Actual Behaviour
If one first toggles the PSLab MIC, the In-Built-MIC feature doesn't work after that, regardless of whether it is toggled directly or after switching OFF the PSLab MIC. This happens only if the PSLab Device isn't connected.

Expected Behaviour
The In-Built-MIC feature should always work, regardless of the fact that the PSLab MIC feature has been used first or not.

Steps to reproduce it
Ensure that the PSLab Device isn't connected.

1. Click on the PSLab MIC checkbox to toggle it.
2. Either turn OFF the PSLab MIC first and then click on the In-Built-MIC checkbox, or click on the In-Built-MIC checkbox directly.
3. One will observe that the feature isn't working.

Screenshots of the issue
This is a screenshot of the OscilloscopeActivity after following the above steps:-

One can observe that there is no plot for the Built-In-MIC input.

Would you like to work on the issue?
Yes.