Great topic. I do think mixing plays a significant part in the success of a stock track. When I started at AJ I tended to mix my tracks as if they were pop releases intended for listening. It was only later that it dawned upon me that I am making background music; secondary content, which for the majority of the time, would be placed behind a voice over or SFX, and supporting some sort of primary video content. This changes the game quite a bit in my opinion.
I tend to mix my music to sound warm and full with a slightly scooped mid range. I do this for two reasons: 1, because bassy music tends to be better received by average listeners, and sounds more “impressive” on small speaker systems, and 2, so that there is more sonic room for the midrange of the human voice in the case of voice overs. Although many customers may not be actively listening for these nuances, they will notice how well the music sits in their projects if they choose to download a preview version.
I also tend to make my tracks quite wide-panned with not too much audio weighted towards the centre. This is because voice overs will inevitably be placed in the centre, so by making room here the voice over will sit nicely with the music. If you listen to many of the all-time top sellers on AJ, you will notice a slight bias towards lower frequencies and a wide stereo image. Coincidence? Maybe so, but I personally think this stuff does make a difference.
I wouldn’t say mixing is the make-or-break factor in the success of a track, but paying attention to these small details (among many others) might just earn you those few extra sales.
Just my 2c