The app I was thinking of is Ace Studio AI . It can split stems and turn Audio, MIDI & MusicXML into sung lyrics. If your files don't have lyrics, you can type them in and it will sing them. It cannot split lyric only stems, however nor will it scour the internet looking for words. There are a ton more features, most of which I will never use.

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See how experts use our AI singing voice generator to create songs with unique vocal styles and distinct sounds. It's perfectly fine to start generating AI vocals from either a MIDI or an audio file. With artificial intelligence, you can explore diverse styles for harmonies and backing tracks. Ideal for both personal and commercial use, this custom vocal generator offers a royalty-free license to elevate your songs.

It has a blue button that says Get Started for Free. Uhhhh... not really. You buy the app then have seven days to request a refund. Time is money on my desk so I paid for a permanent license in advance for the Pro version and got to work.

I fed it a MusicXML file of a choral piece written by my church's organist in Finale. It split the voices as expected. When I hit Play, the voices started singing. That was impressive. Changing the default voice to one of the 80 included on each track and making each into five for a choir effect took a few seconds. Being AI, much of the word pronunciation is terrible but my goal was for rehearsal tracks for my choir. I consider the results outstandingly not horrible. Mind you, had I spent any real time with it, they would have been much better. Anyway, I had five tracks (Mix and Part Dominant for Soprano, Alto, Tenor & Bass) being emailed to my choir in about an hour. I will not be asking for the money back during the next six days.

All rendering and exporting is done in the box, not online and that can take awhile—think video rendering and exporting times. I can't imagine using this without having something as powerful as my Mac Studio M2 Ultra.

You are not limited to the 80 included AI voices and can make your own using samples (!) but I haven't tried any of that yet.

BIAB 2024 Audiophile Mac
24Core/60CoreGPU M2 MacStudioUltra/8TB/192GB Sequoia, M1 MBAir, 2012 MBP
Digital Performer11, LogicPro, Finale27/Dorico/Encore/SmartScorePro64/Notion6 /Overture5