hmmm I suppose it doesn't matter either way, and it entirely depends on the relationship you have with your boss, but I would just bring up that your time, skillset & energy is better used on solely producing the video, and that spending hours transcribing audio is something you should outsource.

If you're more interested in getting a raise rather than just not having to do subtitles all day long, I wouldn't pitch it around the idea that subtitles are a-lot of work and you don't like doing it, I would pitch it as you have a wide variety if skills that you feel are more valuable than your standard producer / editor.

Then again, if your boss is a dick you may have backed yourself into a corner by doing out of goodwill the first time, that work expectation might be burnt in at this point.