How to put my voice recording in a document?

On iOS devices you can record the audio from within Pages. On a Mac, you need to record the audio in a separate app, such as QuickTime Player, or Garage Band (or use an existing audio track), then add it to the Pages document.

See the linked instructions for more details.

https://support.apple.com/guide/pages/add-existing-video-and-audio-tan785873ed3/mac

You can simply drag and drop the voice recording from the Voice Memos application on the Mac — directly onto the open Pages document. This works on Mojave and Catalina where I have tested it with Pages 10. Don't expect this audio to survive export to PDF, or MS Word properly. In the latter, no Word .docx document is created, but rather a folder named for the exported document, with the Word .docx and audio file in it.