Reviving an old thread here..
I did some extended testing and tried to figure what settings worked out the best for _ME_. I turned my Auzentech Home Theater HD audio card into Entertainment mode, hooked up my Westone UM3X earplugs as those are what I'm using with my iPhone when listing to audiobooks and then I took my Harry Potter and the Deathly Hallows CD number 3, picked a random track and started with Apple Lossless version of it and then I tried all kinds of different AAC solutions like the built in podcast preset from Apple, going custom with everything down to 32 kbps (16kbps mono) and up to 256 kbps (128 kbps mono) with CBR and VBR, LE and HE, optimised for voice on.. About anything I could think of, then I sat down and compared them to each other.
The first thing I notice is that HE (High efficiency) is a most when going under 64 kbps (32 kbps mono) in my opinion, if you go LE (Low efficiency) on such low bitrates it WILL sound hollow.
Secondly the optimise for voice option should always be selected, it makes the voice much clearer and in focus. If you go for insanely high bitrates for audiobooks like 128kbps ++ you wont notice much of a difference really, besides the voice becoming more prioritized compared to background audio but on lower bitrates the voice gets muddled with the background noise if this option is not activated.
The difference between CBR and VBR was barley noticeable whatsoever, but VBR normally results in 0.6-0.8mb larger size per track, not much but if you can't really tell the difference why bother? Considering it's like 250-300 tracks for each audiobook this might save you a few hundred megabytes per audiobook.
One thing that really bothers me with iTunes is the complete lack of Sample rate control, we've got the option for auto (gives you 44.100 kHz?) and 32.000 kHz. Normally 22.050 kHz would be sufficient for audiobooks so why we can't opt for this even when selecting HE and bitrates under 64 kbps is beyond me. Considering the included "podcast" setting gives you 22.050 kHz why on earth can't we select this ourselves? That's probably some wasted space right there going down the drain with any really noticeable difference in audio quality.
What struck me and surprised me the most is the difference between going stereo and mono. I was under the belief that audibooks = mono with some very random exceptions making mono a no brainier as it cuts the size in half and if the audiobook is recorded in mono how would stereo make any difference, right?
Well, I don't really know if the Norwegian version Harry Potter and the Deadly Hallow is recorded in stereo or not but the difference in audio quality when going from stereo to mono was staggering to say the least.
A 80 kbps (40 kbps) @ 32.000 kHz HE CBR (and VBR for that matter) mono track sounded noticeably more hollow than a 48 kbps (24 kbps mono) @ 32.000 kHz HE CBR (or VBR) stereo track.
So my conclusion was that my sweet spot was about 48 kbps (24 kbps mono), 32.000 kHz HE CBR STEREO and NOT mono. In fact in order to get something sounding about the same with mono I had to go 128 kbps (64 kbps mono), 32.000 kHz LE (as HE only goes up to 80kbps which sounded more hollow than the 48kbps HE stereo setting) CBR resulting in much bigger files compared to the 48 kbps stereo, HE CBR route.
After this experience it's beyond me why everyone seem to think that mono is the way to go over stereo? But perhaps this has something to do with the Norwegian version of Harry Potter has been recorded in stereo, I don't really know.
