You are probably referring to 'Stereo mix' or 'Record what you hear' depending on the audio device You could try using the 'Record> Classic Wave Driver' in Sound Forge
You should first confirm the input mode in Windows sound settings. The Realtek interface usually list it as "Stereo Mix". I do not know it any of this will work in Sound Forge-3, which I used on the Windows 95 PC about a hundred years ago .. or so it seems.

Sorry, the Sound Forge is Cleaning Lab 3 and is not that old. There is a Cleaning Lab 4 out but I haven't got it. 3 works fine except for the 'Records Output' which just stopped working, I can select it but it will not record anything. I tried to record a piece of old music: Jack Stillman's Orchestra - That's My Girl, if you want to try. I have used it in the past so I know it used to work.

@Kent-Birge Are you using a dedicated sound card or just the computer's built-in audio device?

Open the Windows sound settings. To do this, hold down the Windows key and press R. In the Run dialog box, type mmsys.cpl and press Enter.

The Sound window will now open. Alternatively you should also be able to open this window in the audio settings of Cleaning Lab. Go to the "Recording" tab. A recording device like Stereo Mix or What U Hear must be enabled here. Otherwise, system sound recording is not possible. If you use a dedicated soundcard, make sure there its audio loopback is enabled.

If nothing is displayed in the Recording tab, you can right-click into the empty white area and make hidden devices visible via the right click menu.