That's a good point. There are various xml markup formats for synthesizing text to speech that let you tag words for emphasis and pitch, but there's not granular or expressive enough to mark up individual syllables of words, and it's not useful for singing, for example.

It would get really messy of you had to put tags around individual letters, and letters don't even map directly to syllables, so you'd need to mark up a phonetic transcription. At that point you might as well use a binary file format, not xml.

But for that kind of stuff (singing), there are great tools like Vocaloid (which is a HUGE thing in Japan):

https://www.vocaloid.com/en/

https://en.wikipedia.org/wiki/Vocaloid

VOCALOID5 - Walkthrough

https://www.youtube.com/watch?v=UAtVGHl1AFM

(Check out the "Cool / Cute" slider at 8:22!)

Here's a much simpler and cruder tool I made years ago (when xml was all the rage) for editing and "rotoscoping" speech pitch envelopes, called the "Phoneloper" -- not quite as polished and refined as Vocaloid, but it was sure fun to make and play with:

Phoneloper Demo

The Phoneloper is a toy/tool for creating and editing expressive speech "Phonelopes" that Don Hopkins developed for Will Wright's Stupid Fun Club in 2003, using Python + Tkiter + CMU's "Flite" open source speech synthesizer. It modified Flite so it could export and import the diphone/pitch/timing as xml "Phonelopes", so you could synthesize a sentence to get an initial stream of diphones and a pitch envelope. Then you could edit them by selecting and dragging them around, add and delete control points from the pitch and amplitude tracks to inflect the speech, stretch the diphones to change their duration, etc. It was not "fully automatic", but you could load an audio file and draw its spectrogram in the background of the pitch track, so you stretch the diphones and "rotoscope" the pitch track to match it.

https://www.youtube.com/watch?v=qy5cqV8ypIs