In Japanese, it is spelled/pronounced 「バッハ」. Yup, like "Baja." I always think of the Mexican chain Baja Fresh.
Well a little earlier I did a web search and found a few answers like this one: basically that the Japanese pronunciation is closer to the original German than the way English-speakers pronounce it. While this is certainly possible, I decided I didn't want to take it on faith from Japanese people that their way of pronouncing "Bach" is more correct than ours. 勝負！(showdown!)
After a few searches in English, I came up with this link, which has a little audio clip near the bottom with the proper German pronunciation of "Bach." Basically, English and Japanese are both off.
Not that I can speak any German, but I've heard enough of it in movies and such that I can somewhat pick up on pronunciations. The "ch" sound seems to be a more guttural union of "h" and "k" than we use in English. Almost a soft "k," if that makes sense. If you've studied any basic German, one of the most basic words is "ich" (I), and it has the same kind of sound.
It's not a sound we use. Japanese doesn't have it either, and so while we English-speakers decided to just go all in with a hard "k" sound, the Japanese dropped the "k" and went full "h."
The "ch" in "ich" and in "Bach" is pronounced completely different, it doesn't sound the same at all. I can't really describe it, but at least for a German person the voice samples show the difference quite well. Bach: http://upload.wikimedia.org/wikipedia/commons/f/fb/De-Bach.ogg http://upload.wikimedia.org/wikipedia/commons/4/43/De-ich.ogg
I don't see where a "K" is supposed to hidden in the German pronunciation. It is however my impression that English speakers often substitute a "K" in place of the "ch". ;)
Now Japanese people have a whole lot of other problems with the German language, oddly enough also with the distinction between the pronunciation of "auch" and "auf". But they usually can manage with a few minutes of pronunciation training. :)
I also believe that Englsih speakers map the German "ch" sounds to a "K" because it's the closest thing they got. :) That's usually how people start listening to and speaking a foreign language and it's where all the funny accents come from.
Describing the pronunciation it really hard for me though, for one because English is not my mother tongue and second because I would even have problems describing it in German. ;) If I absolutely had to describe it, I would reluctantly say that it might relate a bit to how it's pronounced in Japanese and at the same time that the "ch" sounds a bit like the "wh" in "cool whip" (or was it whipped cream?) when Stewie from Family Guy says it with his odd "hwip" pronunciation, albeit more roughly expressed.
I can't really say if the term "guttural" describes the pronunciation. I would say that the "ch" is aspirated, with a lowered jaw, a stiff and raised tongue and relatively closed lips, pressing the air out slowly. Is that guttural? :)
So then it appears that the Japanese does edge out the English pronunciation! And if I ever take up German, I'll have to work on my "ch"'s.