That's because British people are awesome! 
And arrogant lol. 
That's rich coming from an American (joke!). Anyway, I prefer to think of it as national pride.
For some reason I always imagined Visser Three with a German accent. XD
Ha ha! I guess somebody has been watching too many Indiana Jones and old Bond movies eh? I'll always "hear" Visser Three as a Nazi now!
And of course, the Hork Bajir are Scottish.
lol, awesome! I never thought of them that way, but I still think it's hilarious! "Jara Hamee eat Haggis!"
Okay enough of that, I'll go through what I remember them all sounding like. I guess I never really thought about it before now but I'll give it a shot:
Human Animorphs: No specific accents really, but they did all sound slightly different in my head. I know they should probably all sound American at least, but they never have done to me. Sometimes their "voices" sounded different depending on what mood they were in.
Ax: When using thought speech his voice was kind of neutral. Not robotic or anything, just very calm and well spoken. In his human morph, the way he talked reminded me of the way a foreigner talks when they don't know English very well.
Visser Three: Again, not any specific accent, although he does sometimes remind of a Bond villain, lol. I always imagined his voice to have a very arrogant sounding sneering voice, but most bad guys sounds like that to me in my head. Very annoying.
Hork-Bajir: Hard to describe in terms of accent, but sort of like a primitive tribe people type of voice. Like a cave man who's learned to talk (no disrespect intended to the Hork-Bajir, that's just how they sounded to me). Toby's voice I imagined as more normal sounding due to her intelligence and use of language.
Other Andalites: Varied a lot depending on the individual. The normal Andalite voices I thought of as similar to Ax, but the arrogant b*stard Andalites we met sounded more like Visser Three. Arbron sounded like a cross between Ax and Marco, and Estrid was more like a cross between Ax and Rachel/Cassie.
