The addition of opensl has really brought on a lot of playback issues that I've had to overcome. The issues are 100% the fault of the manufactures of the device, but I was still able to find a work around.
Issue #1: Some devices like the galaxy s2 and galaxy note decode mono mp3s incorrectly. Instead of decoding out to mono, they decode the audio into stereo, with the same data in the left and right channel. Doing this is technically okay as long as they let the user of the decoder know the output is actually in stereo instead of mono. These devices do not. They report that the output only has 1 channel instead of 2. GMMP would try to play stereo data in mono, which would result in playback being slower than normal. I wrote some code to detect this behavior and it works fairly well, so I believe this is completely fixed.
Issue #2: Adding or removing of audio samples in the beginning of the decoding process. I have 3 devices that run ICS and on all 3 of them, the mp3 decoder would add extra samples of silence to the beginning of the audio data. This interferes with gapless playback. For AAC playback on the galaxy s3, the decoder would completely remove the first 1000 or so samples of audio, which of course interferes with gapless as well. The other 2 ICS devices do not do this, so there is a inconstancy between the opensl implementations on ICS and newer devices. I did find a workaround that works on the 3 ICS devices I tested on and should be versatile enough to work on most other ICS devices.
If you run into either of these issues on 1.3.8, you can turn off opensl through preferences -> audio -> enable opensl. Please send in an email if you do experience the issue.
You may ask why OpenSL is even used if it is causing so many problems. The reason is the non opensl mp3 and aac decoders are not guaranteed to be on every device. 99% of the devices tested do contain the decoders, but with 4.1 removed the AAC decoder that was being used. OpenSL may be buggy but it is guaranteed to be there on every device running 4.0+
-Fixed scrobbling issue with shuffle all enabled
-Prefetch errors will cause the track to skip immediately instead of after the 5 second timeout
-Fixed playback speed of mono files on galaxy s2, note, and possibly other devices updated to ICS
-Album search now will show albums that match the album name, artist name, or album artist name
-Album search will now only show an album once for various artist albums
-Fixed issue where positions in file browser would be lost when changing views
-Selecting 'Play Next' will now cancel 'Shuffle Collection' if enabled
-Fixed crash when updating notification controls
-Caught out of memory exception when trying to load background art
-ICS lockscreen will no longer disappear after pausing playback
-Changing tracks when paused/stopped will no longer automatically start playback
-Added chinese translation
-Updated russian, czech, spanish and german translations
-Fixed gapless playback for 4.0+
-Fixed issue where there would be a gap between the first and second song when playing a folder
-Fixed crash during file deletion