[popcornmix]

As for the AV syncing - which is the best mode to use for the PI or this particular case? I was trying audio clock which seemed to work fine, did only a short test though. And do you know if AV sync will only kick in if refresh rates don't match, or is this thing always on once enabled in settings?

My understanding is that with dvdplayer, "adjust display refresh rate..." just switches to the best refresh rate.
If your file has perfect timestamps (e.g. 24fps/48kHz stream contains exactly 24 frames per second and 48000 audio samples) *and* your graphics card's video clock is synchronous to your sound card's audio clock, then everything should be good.

However neither of these conditions are generally true. You need another feedback path that detects drift in audio/video and compensates.

On dvdplayer you need to enable "sync playback to display". That gives 3 sub-options:
audio clock : The clock just jumps in software. I believe this is the same as having "sync playback to display" disabled and that will still have video judder
video clock (resample audio): For non-passthrough audio is resampled to fit in. Video should be smooth, but audio quality may be (marginally) worse. For passthrough drop/dupe occurs.
video clock (drop/dupe audio): audio frames are dropped/duplicated to fit. Video should be smooth, but audio quality may be worse

Resample is more expensive in cpu than drop/dupe but is probably less audible.

omxplayer has an additional sync scheme where it has a separate PLL for the HDMI video clock, and can maintain sync without harming audio or video.
This is only enabled when "adjust display refresh rate..." is enabled, so manually choosing 24Hz won't be as smooth as automatically switching.
This scheme is much better than the others, and everyone should enable it.

omxplayer can also support audio resampling. If "Sync playback to display" is enabled (with any sub-options) it will behave like dvdplayer in "video clock (resample audio)" mode.
This can be useful if you get audio dropouts in "adjust display refresh rate..." mode.