I have the same problem. This is the answer from Apple Support. Link is in Spanish but you can change that. Personalizar los recuerdos en la app Fotos en el iPhone - Soporte técnico de Apple (ES)
Look through the following link you can consult these more detailed functions and how you can edit them.
Once the slide is created, it will be saved as a "memory".
And to change the duration of a memory depending on the number of photos in a memory, you can change the duration of a memory to short, medium, or long. The option you choose automatically adjusts the number of photos included.
1 Open the Photos app on your iPhone.
2 Scroll down to Memories, then tap a memory to play it.
3 Tap the screen, then tap .
4 Tap Short Duration, Medium Duration, or Long Duration.
If you have customised the length of the video by adding or removing photos yourself, the selected length is personalised.
In my case my number of photos is 43 photos and 6 videos. I only have the possibility to choose “short” or “medium”.
Regarding the issue “music” I have no solution as I didn’t ask for it but I am sure Apple Support will be able to help.
If you have the solution, please add it here.
Thank you and good luck!