Access to Apple Music Library In Photo Memories

To the right of the word"Music" (below the Done button" in your screenshot is a looking glass. Tap the looking glass to open a search. This will let you browse your Music Library.

Can you use any of the songs there? You should be able to use all songs from Apple Music.

When I click the magnifying glass icon, m music library is shown nd I can ear for a song I want. Which system version is running on your iPhone? Do you have an  Music subscription? I can only pick the songs from  Music, when I want to use a different soundtrack.