Media players

Do media players have a sound?
A lot of audiophiles prefer Foobar, they consider it the player with the best sound.

According to the developer Peter Pawlowski:
Does foobar2000 sound better than other players?
No. Most of “sound quality differences” people “hear” are placebo effect (at least with real music), as actual differences in produced sound data are below their noise floor (1 or 2 last bits in 16bit samples). foobar2000 has sound processing features such as software resampling or 24bit output on new high-end soundcards, but most of the other mainstream players are capable of doing the same by now.
http://www.foobar2000.org/FAQ

If we assume that a media player is program having an interface allowing you to pick a song and send it to the OS audio, there is little reason to expect any difference in sound between them because OS audio=OS audio.

There are claims that even in this case they can sound different.
The reasoning is that any electrical activity going on inside the computer will affect sound quality. Probably because more activity=more disturbance of the DA of the sound card (jitter). RFI and PSU are often mentioned.

Often a bit more is done than sending a file to the audio device only.
Sample rate conversion (SRC) is a nice example.
The XP K-mixer is well know for degrading the sound quality the moment it kicks in.
OSX had some issues too.
dCS did some measurements.

It all demonstrates that the way DSP is implemented is crucial for the results.

You do have to choose your algorithms carefully, program it correct and very important, the variables used should have the right precision.
Roughly speaking, a 16 bits audio path is inadequate, any DSP will degrade de sound probably due to quantization error.
32 bits is fine, most major OS use it. The artifacts are small and probably not audible most of the time.
Best result needs a 64 bit audio path. A lot of pro-audio software uses this.
Even when doing many calculations (exactly what happens when producing audio) the precision is such that any artifact is well below the noise floor of top quality gear.

64 bits and still artifacts? Yes, everything on earth is finite. Analogue components don’t have a infinite precision, it ends definitely in the thermal noise. Likewise DSP has a finite resolution but as long as it remains below the (thermal) noise floor we won’t notice.

Drivers can make a difference too.

There are horrible examples of drivers incapable of delivering bit perfect output.

The fun about computer based audio is that you can simply try a lot for free.
Download Foobar or J River Media Center (my personal preference) and do a listening test, try different drivers, etc.

But don’t forget the ergonomics, it is the tool you will use to handle your audio.

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