General infoAndroid has serious trouble with low latency audio - most of the time every sound is played with 130ms or greater latency. If the latency is the same every time the audio is played - it's easy to sync to music. Otherwise you won't be able to have a constant perfect sync. This is what's happening on some small number of devices and is because of different implementations of android audio sub system and/or very specific hardware used.
I am aware that he game has music sync issues on some of Xperia phones, Nexus 6 phones (Nexus 5 seems to be ok) and NVIDIA SHIELD tablets (K1). There are some things you could do to play BeatX with music in sync so if you are interested - continue reading :)
NOTE: Since v2.4.2 there is a new button in Settings/Advanced/Sync approx. - try different settings and recalibrate using Settings/Set delay. The NEW(15x) value should help in some cases (this fix was coded specifically for (my) NVIDIA Shield K1 Tablet which had timing issues and is now working perfectly and constantly on all songs)
Xperia phonesThis issue is likely to never be resolved (from my side) as Xperia phones are known to have trouble with music playback even in their build-in music playback app. This is most likely to be caused by hardware problems or errors in custom Android build , or Sony's audio drivers - i really can't do nothing about this (* SEE EDIT BELLOW)
What is happening here is that there is a skip during the song playback. I could do a re-sync but i just cant determine when because the current music position retrieved from media player is never (not even on devices where BeatX has perfect timing) the same as the time that really passed since the music playback started (e.g. what the step chart is synced to)
There are many articles describing this Xperia issue so check this out:
Nexus/Google phonesGoogle attempted (and failed) low latency audio on nexus phones. As a result - nexus phones behave differently than 99.9% of the phones out there when it comes to music sync.
BothI could probably fix this but i need phones to test. Considering a small market share this represents + the costs - i just won't do this unless some phones miraculously fall in to my hands. If you have one laying around (could be damaged with partially cracked, but working display, without wifi, poor signal coverage what ever), feel free to send it to me and contact me via email@example.com
* EDIT: I have reports from a user that XPERIA Z ULTRA (which had problems with factory Android) is working fine with Cyanogenmod. This means that the issue is most likely caused by Sony's custom Android build or one of the pre-installed sound enhancing aps.
If this is the case - you can TRY to fix this by disabling all the sound enhancement features in your Settings/Sound (let me know if this helps)
You can also try to install Cyanogenmod on the nexus devices.
Some users also reported that converting the music file from OGG to MP3 (or MP3 to OGG) or even WMA helped the sync on devices where sync was different every time they played a song. You just have to keep the same bitrate, quality e.t.c. You also might need to re-sync the step chart because conversion to a different format might shift the start of the song a bit (few ms). It's a bit too much work to do but can help if you really want to make it work.
*EDIT 2: I now have bought NVIDIA Shield Tablet which has timing issues in the latest Android 6 update. This means that i finally have a device to test this and there is a high probability that you will see some updates on this issue! One of the things i figured out here is that Announcers can mess up your timing. I was able to improve the engine so it works in 90% of the cases but it still needs some work/testing. Stay tuned for future updates!
Let me know if this helps via firstname.lastname@example.org