Well, I intentionally didn’t try to justify it because I don’t want to have to get a rebuttal from Kakashifan72 who has been studying japanese for 6 months and never listened to an audiobook in japanese in his life but is very certain about their value. I just wanted to make it clear that while I am saying audiobooks helped me a lot I’m not trying to preach them or suggest other people do them as well and if anything its the opposite.
As morte said, podcasts and audiobooks are completely different. Podcasts are better.
Since you’re asking though, to quickly sum it up, if your goal is to listen to audiobooks then listen to audiobooks. Otherwise, your listening skill will be used for spoken japanese, right? This is where the problem comes in.
Audiobooks are not normal spoken japanese. They are slower, cleaner, clearer, and literally use different language. Its an artificial environment that makes the hard parts of spoken japanese easier and makes the easy parts of spoken japanese harder.
Easy parts of spoken japanese (low vocabulary and grammar requirements) → needlessly has required vocab and grammar increased
Hard parts of spoken japanese (speed, mumbling, inconsistency in thoughts and general quicks) → slow perfectly spoken and constructed sentences
You lose your chance to practice the hard parts of speaking and have that replaced with required effort towards things that won’t come in handy in spoken japanese. Whereas if you practice listening to spoken japanese…then 100% of your effort is directed towards things relevant to spoken japanese.
As a result, its subpar. Far from useless, but not the best. Like I said, I used it (and still do) as a way to index words and their audio in my mind. For example, I didn’t know 苦労 the first time I heard it. I have seen 苦労 a million times and would never mistake it, but I had literally no recollection of hearing it since I never practiced listening for 4.5 years, so I couldn’t make the connection. In my case, however, once I hear a word once and can associate the kanji with it in my head, I’m basically guaranteed to be able to recognize it from then on out. So its just a matter of hearing and recognizing every word once. Audiobooks slow pace and clear pronunciation and familiar usages of words in certain contexts gave me plenty of chances to hear a word and flip through all the kanji combinations with that reading I could think of in my head that fit what I expect in that context until I “saw” the right one. From then on out I would have it on faster recall. So for me, it was a nice controlled environment to expose myself to all these words so I could index them in my head with an auditory cue. After like…7 audiobooks I’m at the point where I’m listening to them on 120% speed and I’m going to be done with it after I finish the series I’m listening to because its already run its course honestly. I’m just liking this series and wanna finish it. Efficiency wise I should stop now imo. Don’t even get me started on nonfiction audiobooks, though, honestly. I listened to two of those and they don’t even count. Shit was a joke how easy it is compared to actual full speed native conversation. Those actually didn’t feel like they helped me really.*
*I’m sure you can find some harder nonfiction, but I certainly didn’t.