Gear S3: How to play music from phone

[Swifty]

When I open the music player, and change the music source to phone, all I get as content is the word "Unknown".

I have hundreds of albums on the SD card in my S7 Edge, on the SD card, so why can't the watch find it?

I could perhaps download a few albums from the phone to the S3, but then I would be continually juggling albums.

[AntS]

Do you get the same "Unknown" message for music files stored directly on the phone itself, @Swifty?

[Swifty]

No, tracks on the watch appear/play correctly.
As far as I know, the tracks on the watch are in the expected place.

[Swifty]

Additional information:

Sometime, a few months ago, I was using the ES File Explorer,  and was asked to use a different path to reach directories on my SD card. I didn't fully understand the issue, but followed the instructions, and my access succeeded. But this has left me with an un-intuitive path to my SD card...

Perhaps there is some sort of access permission required?

[Swifty]

How odd. Today, I can switch to my phone as the music source, and it finds just one track, one that I downloaded to the phone yesterday. It doesn't play though - nothing happens, not even an error.

Yesterday, I switched to my phone as source, played the track that appeared, and it played... on the phone!