There are quite a lot of different aspects to it, like how you actively pay attention to language with it or keep a sentence/word in working memory.
After seeing the benefits of verbal reasoning, I have heavily investigated ways to improve this, which as a side effect helped me learn a lot about my inner voice.
I can understand this. I get a lot of earworms from music replaying in my head. This is one of the most difficult things to stop because it occurs subconsciously. I have found meditation to help. Particularly, the kind where I count my breath while actively focusing to avoid idle thoughts/sounds. When I do this I get less frequent earworms. I have also found to actually stop them, I need to pay attention to them and then drown them out. Often this leads to replaying them however.
It’s also possible to simply focus on a song without a voice to displace the other song, like with your mantra.
Lately, however, I have been slightly suffering from replayed conversations, when I am speaking to someone for a while and then later during the day my brain replays random parts in their voice. This can be quite annoying.
Although it isn’t unpleasant when it is sounds and voices I like, it can be distracting when working.
A month ago, I did some interesting meditation that completely shut off idle thoughts. I was also interestingly more able to focus after that then I would have been on default. However I need to rediscover exactly what I did back then as I haven’t gotten the same result (to the same intensity) again yet.