When I read the manga I pretty much hear a variation of my own imagined voice for Guts and Casca mixed with the voices they had from original 97 anime in Japanese...expect that they are speaking English instead of Japanese (the sound and rhythm of their voices mostly), though for Guts he has a bit more of guttural/gravel quality to his voice, especially post eclipse.
Casca I was really taken with her voice in the original 97 anime in the Japanese tract, to me it was perfect it just really captured what I felt she should sound like. But I imagine her voice to but slightly less bitchy and shrill (when pissed off) sounding and poised/dignified when she is speaking normally, even when she is in full commander mode in my head I hear her as down to earth and feminine but also firm and authoritative. I liked the English dub voice for her OK, but when she was trying to sound tough or commanding she came off...I don't know what the word for it is, but too abrasive...I guess...also I always hear them speak with an American accent. When Casca is making her intelligible sounds in her current state I deliberately tone it down in my head so it's less infantile sounding. Hearing her articulate the deranged baby sounds she makes in the the last movie and the the new anime made me cringe so hard.
It is funny for Schierke I imagine her having a very soft calm voice that has a squeak to it occasionally, kind of like Sweetie Bell on
MLP.

As far as hearing different accents in my head for other characters, the only ones I get a different accent for while reading their lines is Azan and Farnese. Azan is the one I imagine for some reason as having a sort of thick Scottish brogue for some reason. And Farnese I can't help but hear her with a refined almost posh sounding British accent.