First the UIFlow UI does not show a "play WAV file" not in the SDCard section, the Speaker section, nor in the DAC section.

Second

from wav import wav_player
wav_player.playWav('/sd/myfile.wav')

fails with an I2S does not have MODE_MASTER error.

Third

import machine
dac0 = machine.DAC(25)
dac0.wavplay('/sd/myfile.wav')

does not work either, no sound is produced.

However the similar (according to the code)

from wav import wav
ww = wave.open('/sd/myfile.wav')
sample_rate = ww.getframerate()

# read all data, play in background
data = ww.readframes(ww.getnframes())
dac_buffer = array.array("B", data)
ret = dac0.write_buffer(dac_buffer, sample_rate, wait=False)

works fine. Finally !

If the sound file is big you can also use

while True:
  data = ww.readframes(2048)
  if len(data) > 0:
    dac_buffer = array.array("B", data)
    ret = dac0.write_buffer(dac_buffer, sample_rate, wait=True)
  else:
    break

or similar (notice the difference in wait=False or True)

I hope this helps future users.
In the end digging into the code of what M5Stack provides or not has been the best way to understand how to make the best out of the codebase.

(could not use UIFlow 2 due to SDCard issue)