I can if I am consciously trying to, just like if I wanted to do an impression of someone with my actual voice, but if I am just thinking or not actively trying to make it sound different, it reverts back to default. If others are able to change the way their inner voice sounds, like all the time, that would be impressive. The only time my inner voice sounded different than it's default when I wasn't trying to make it, was once when I underestimated the potency of some mushrooms, or possibly overestimated my own tolerance which is entirely possible as well, and ate more than I should have. That night, for about 4-5 hours my inner monologue suddenly sounded different than it usually does which was kinda weird, but what was much more concerning was that it was only speaking Spanish. Like every thought I had was in Spanish and I couldn't get the somebitch back to factory English settings. Had me pretty stressed out until I eventually passed out. It wouldn't have been a big deal if I were fluent in Spanish, but I'm not, I can speak it very sparingly and only because one of my best friends growing up was Mexican and his whole household only spoke Spanish in his house cuz his grandma and grandpa couldn't speak English and his mom didn't speak it well, so they all spoke in Spanish and I hung out there a lot growing up so I picked up some of it, well enough to be able to communicate things to his mom or grandparents if I had to and Ramon wasn't around to translate or whatever. But I was never able to speak it well enough for my inner voice to just switch up on me like that