I think that with the music mentioned in the codex for Memories of a Duet, Mythal was the composer, and I think Mythal was the pianist. I think Solas was the one watching. The reference to godhood (mindless worship) does not seem to fit Solas in my opinion, because the first thing that the Inquisitor runs into in Trespasser is that mosaic memory door thing that says very specifically that Solas is not a god and has never been a god, and there are other references to Solas in the Crossroads and Lighthouse where he is being referred to as "Wolf" and "Wolf Lord" (and Solas). Not really a god though, not by his people. And I cannot imagine Solas would have put up with mindless worship for long, and neither would Felassan. I suspect that he did probably play, but that music was never his passion as art was.
Also, the card for the codex entry has an image of Mythal, and it could have had the one they used for Solas things (the wolfy one), and it didn't. I think that piano was in the conservatory not because Solas played, but because Mythal did, and it had been placed there in the hope that she would join him at some point. And then the room got filled up with whole lot of Inquisition memorabilia after that. Which is truly sad to me, because he had like one thing in that room of a happier time for actual millennia of existence, and then everything else is Inquisition era stuff. And that was like. A year. And the murals are mostly Inquisition murals. The one mural that is straight Mythal is hidden behind crates.
Memories of a Duet
The sheet music left by this instrument appears to be for a duet. It has been annotated by an expert hand.
The annotations are accompanied by clear emotional impressions: diligent practice, with a ruthless eyes to mistakes, the relief of private achievement, away from well meant misunderstanding and mindless worship, an unspoken joy in the center of rising, perfect echoes.
Finally, a beloved memory surfaces. A smiling glance, meeting at a crescendo, a shared moment of understanding, seeing completely and being wholly seen.
I also think that Mythal was the composer and player because one of the Caretaker's memento's is Mythal's grace, which speaks of a medallion with Mythal's symbol and music again.
Mythal's Grace
A medallion bearing the symbol of Mythal, ringed with the notes of a song inscribed in spectral notation, music that cannot be heard in this world.
The spectral notation is also really interesting to me because it seems to imply to me that the music was made for spirits. Did they play music together before Solas took a body? To me, Memories of a Duet are two memories, the first after Mythal ascended, practicing and composing alone, and I think closest in time. And I wonder if the last memory of eyes meeting is before Solas left the Fade and before Mythal became a god. I wonder if Mythal composed the duet based on that memory. I wonder if Mythal missed that too. (I suspect so, whatever there had passed between them.)
My guess is that Solas played the elven lute? bass? whatever the stringed instrument is? while Mythal played the piano, because there is one there too, and there was, if I remember correctly, one in his quarters in Haven.
I bet he played the string accompaniment to the piano alone after Mythal's death and was just sad about it, and it did not at all help in making him more centered and better able to conduct a ritual to seal the Evanuris away.
Also, I think it's the only interpretation that really makes sense to me across all world states.