Hi Beebs,
I recommend the following procedure:
Create a separate Registration Bank for each MP3 song (in which the MP3 is memorized on Registration button #1). The remaining buttons (2 - 8 ) of the bank are either free (so they don't light up), or you use them for alternative keyboard voice settings (for this one song). I have already described above how registrations that are not required can be deleted (so that the relevant buttons no longer light up).
Name each Registration Bank file with the song title that will be played with it.
Store these Registration Bank files either all in one folder (e.g. "Registrations") or, if desired, in sub-folders by first letter (e.g. "ABC", "DEF", etc.) or in sub-folders by genre (e.g. "Country", " pop", etc.). It is important that YOU can quickly find any Registration Bank, i.e. any desired song title, at any time.
If you load the Registration Banks directly, you always have to manually press Registration button #1 after loading the relevant Bank file in order to load the relevant MP3.
Note: If you are working with the Playlist function, i.e. adding the previously created Registration Banks to one or more playlists in the desired order, you can specify for each playlist entry (= song title) that when you tap on the song in the playlist not only the relevant registration bank is loaded, but also automatically a specific registration from this bank. In your case you specify that Registration #1 is always loaded: In the Playlist display, touch "Edit" and select the desired Registration (called "Regist Memory" here). See also pages 90/91 in the SX700 Owner's Manual.
By the way, I recommend that you save all of your own data on a USB stick and regularly copy the content of this "main stick" to the computer and external data carriers (e.g. external hard drives, other USB sticks, cloud storage services). Of course, you could also work with the internal user drive, but creating backups here is much more complicated than using USB sticks. Each file type would have to be copied separately to a USB stick. In addition, the file system of the internal user drive is not compatible with USB sticks, since the system uses a fixed main folder for each file type in the user drive and the user drive also has a different drive letter. Registrations and playlists will therefore no longer work if the corresponding files are copied from the user drive to a USB stick (or vice versa).
To bring up the User drive in a File Selection display, touch "User > User". To access the connected USB stick, touch "User > USB1".
Important: When working with Registrations and Playlists, only ONE USB stick should be connected at a time. If several sticks are connected at the same time, there is a risk that an "incorrect" drive letter will be memorized in the registrations/playlists (and the registrations/playlists will no longer work later if the sticks are connected in a different order or if only one stick is used).
Hope this helps!
Best regards,
Chris