Using Mac Dictation
Now that you have Dictation set up, it’s time to start using it. Go to a document or other text field and place the insertion point where you want the dictated text to appear. Press the Dictation keyboard shortcut, and start talking.

When your Mac is listening it will display a microphone to the left or right of the document. If you turned on advanced Dictation commands the microphone will show up in the lower-right corner of the screen, and you can drag it to another position.
Like Siri, Dictation adapts to you. The more you use it, the better it gets at understanding your voice and accent. When you’re done dictating, click the Done button below the microphone icon. Or, press Fn once, or switch to another window.
One warning about Mac speech dictation: it mutes the sound for other apps. You can get fooled by that if you accidentally leave speech dictation (the little mic icon) on. Then, if you attempt to watch or listen to something somewhere else, you will not have sound. It took me a few minutes to figure out why I had no sound in a different app – I had previously left speech dictation on in a previously used app and it was on another one of my Mac’s 3 displays.
I’m trying to convert a lecture from an MP3 file into text using Pages. Is this possible? Have tried without any luck. Thank you very much for your help.
To do it manually, you would be able to use dictation on the Mac if you spoke what the lecturer was saying. For audio transcription you would need specialized software. One example I found is called Descript.