Davebois, for the most part it is obvious you have really been trying hard to keep the noise down, but singing scales and practicing for an hour seems over the top.
You may not hear him in another room but if he is directly above someone trying to work, the irregular sounds might be going straight down.
Practicing singing is just the same as practicing any other musical instrument and could be just as annoying. You can get a practice pad for drumming but nothing I know of to mute a voice to others.
You did not say your flatmate had a ‘good’ voice but had some singing training. Even a good voice singing scales is not entertaining.
Most of us will turn off professionally made music in the first few seconds if it is not the type of music we like, but your neighbours do not have a switch to stop it.
Possibly your neighbours are keeping their own sound diary, and maybe even recording the ‘singing’.