So I ordered the audiobook from my local library and started listening.
I am sorry to say, Dear Readers, that it's true: Tazaki Tsukuru and all other characters in the novel are characterized with artificial Japanese accents. (One notable exception is Kuro's Finnish husband, whose voice is read in an attempted European accent.) The reader, American actor Bruce Locke, is of Japanese ancestry, but was born and raised in the United States, and to judge from his other work speaks without much of an accent. So why impose a Japanese (or rather, pseudo-Japanese) accent upon the dialogue in the narration?
If the story featured Japanese characters in non-Japanese settings, where they would be speaking in languages other than Japanese, one might still understand the decision to voice characters with an accent. But in this case most of the plot takes place in Japan, and the characters speak unaccented Japanese to each other. No accent is called for.
I can think of only one explanation, which is that someone -- the publisher? the producer? the actor? -- decided that the book was otherwise insufficiently "Japanese" and wanted to ensure that the reader wouldn't forget that s/he is listening to a Japanese novel. This is no small irony, given Murakami's widely acknowledged position as a writer who just happens to write in Japanese and who has spent much of his career deliberately seeking to undermine the cliches surrounding modern Japanese fiction.
Beyond this, it is really discouraging to think that in this day and age somebody thought this was a good idea. Does Shakespeare have to be performed with phony British accents? Does a performance of Molière require actors to sound like Inspector Clouseau? Or is it because Japan is more "exotic" that Tsukuru, Sara, Ao, Shiro, Kuro, and Aka are voiced as Japanese-y caricatures of real people? I used to think that only Hollywood did this, but clearly I was wrong.