audio recording of videocast on iMac OS 13.3

you can use QuickTime Player to do that. but to record the mac's internal sound with quicktime you need additional software. i use the free BlackHole Virtual Audio Driver. then for a screen recording, you can choose "BlackHole 2ch" in the recording options instead of "Built-in Microphone."

or for an audio only recording, choose "BlackHole 2ch" in the audio recording controls.

you will also need to set up a multi-output device in your audio MIDI setup utility to be able to both hear and record the internal audio at the same time.

to set up a multi-output device:

Open Audio MIDI Setup: (found in /Applications/Utilities)

hit the '+' button in the bottom left corner and select "Create Multi Output Device"

then in the panel that appears on the right, in the Multi-Output Device tab, select "Built-in Output" AND "BlackHole 2ch". (and, if you want, any of your other connected devices.)

once you do that, (and BEFORE recording) you can choose "BlackHole 2ch" or "Multi-Output Device" in your menubar "sound options". 

to both hear the speakers and record the audio at the same time, choose "Multi-Output Device". before choosing multi-out, set the internal speakers' volume to a comfortable level first. you cannot control the volume once you are in multi-out mode.

and I should have added, once you set it up, you can open QuickTime Player and use Apple Menu > QuickTime Player > File and choose "New Audio Recording" to start a recording.

how can you compress the file so it can be sent via email or messages?

1. Video is large, accept that. I would advise that you don’t compress further. It is already compressed. Instead upload the video to a temporary hosting site, then include a download link in your email (or have the hosting site do that). One example of such an external service would be WeTransfer.
2. In general, you have no knowledge over what the email limits are for various recipients. Your sending limit may be 10 MB, but the recipient may have a 5 MB message limit. It varies by provider. And there is also a limit to the mailbox total, and you have no way of knowing how much room is left. My stance: avoid big attachments. There is always a better way.
3. If you are going to compress further, then you will have to sacrifice frame size and/or quality and/or the runtime. To get e.g. an 80 MB file under 5MB, you will have to discard a lot of movie information. From very nice to postage stamp size. My stance: avoid video attachments.

sorry, IDK. that's not something I do. maybe someone else here will be able to answer that for you. :)