In principle, a relevant audio processor is not easily implemented in mobile devices as their battery consumption is the main interest when designing devices and applications for them. Some functions indeed should be integrated into a player, like compression and equalisation other than the native (alsa?) audio that is implemented in android. But any realtime processing other than just a simple FFT and matrix (call me equalizer) should be consuming so much battery that I don't know whether it worths the time. Hans knows best on this one.
Since that matter came into discussion, there are people (me included) that are truly frustrated by the constraints implemented into latest androids regarding maximum audio levels. Claiming that they "protect you hearing", most devices have limited their headphone reproduction levels down to minimal. Of course, that serves manufacturers well, as they don't have to implement a discrete high power audio amplifier inside their devices thus reducing costs, but will result in frustration od the user when a recording having wide dynamic range is reproduced through that system.
Stereo tool utilizes algorithms that could be trimmed down for maximum CPU efficiency, while not necessarily serving exquisite quality when e.g. it performs multiband processing or it performs advanced clipping. We just allow user to set a maximum audio level (e.g. -2DBFS) and then we use an AGC and maybe a 3-band compressor and some advanced clipper routines to keep audio level as close to preset dBFS level as possible without juicing off the battery. Quick adjustments like in basic mode should be all 13-year old kids should need to adjust volume levels to their needs, while protecting their ears.
I believe that this could sell well as almost all new phones and tablets could use such audio normalisation. Heck, even Google could be interested in such a project since in EU it is the LAW that audio levels in headphones should be kept below some preset audio level described by some EU standard I cannot recall right now.
https://www.google.gr/search?q=Maximum+ ... one+levels
Many ppl trying to get their phones to sing louder.
tl:dr
my thoughts is that a paid music player that promises maximum audio levels within EU limits could sell well on android.