Q: internal micrpohone not working

a) Which iMac do you have? Look in "About this Mac" -> System Information, in the "Model Identifier:" field.

b) Have you ever had it repaired or the hard drive replaced?

Open the Sound preference pane and select the Input tab. If any other sound input devices are shown in the device list, what are they?

You know then that one of the critical items when replacing the hard drive is ensuring that the microphone wire is not damaged and reconnecting it during reassembly.

After you replaced the hard drive and microphone, did you test the microphone immediately after reassembly?

You created an "aggregate device" as described here:

How to combine multiple audio interfaces by creating an aggregate device - Apple Support

Please delete that device and see whether there's an improvement.

Make a "Genius" appointment at an Apple Store, or go to another authorized service provider. You may have to leave the machine there for several days.

Back up all data on the internal drive(s) before you hand over your computer to anyone. There are ways to back up a computer that isn't fully functional—ask if you need guidance.

If privacy is a concern, erase the data partition(s) with the option to write zeros* (do this only if you have at least two complete, independent backups, and you know how to restore to an empty drive from any of them.) Don’t erase the recovery partition, if present.

Keeping your confidential data secure during hardware repair

Apple also recommends that you deauthorize a device in the iTunes Store before having it serviced.

*An SSD doesn't need to be zeroed.