One of the biggest issues today with smartphones is battery life, and for the better part of their existence it has been the same. It's not the most important factor in a smartphone for everyone, but for many the key detail in narrowing down the wide array of options these days is and has always been longevity when it comes to the battery life of the smartphone. Manufacturers think this way too, customers aren't the only ones, and Qualcomm highlights some ways they have done this with their Snapdragon 801 CPU while using the OnePlus One as an example of what battery life can be like. They focus on the Hexagon DSP capabilities of the processor during music playback to show just how long the battery can last on a single charge, and it's actually quite impressive at 60 hours, however it was with some pretty specific factors so you likely won't be able to re-create the same scenario without following the steps they took or with many other functions than music if any.
The Oneplus One was used for music recently by Qualcomm with the audio being routed through stereo speakers using the audio jack, and on top of this Qualcomm also had all the connectivity radios in the phone disabled(I.E. WiFi, Bluetooth and cell network)so that there was next to nothing to drain the battery except for perhaps the screen and the music playback function. The One of course, does have a decent battery inside at 3,100 mAh so that certainly had some part to play, but Qualcomm is keen on pointing out that this length of time in use is due in large part to the music being re-routed to the DSP instead of relying on the processor to handle the music playback function stating that "This allows the apps processor to sleep while the DSP decodes the audio and sends it to the codec."
The DSP according to Qualcomm uses much less battery power for things like this, and when you have a 3,100 mAh battery on board to help with the task, it's not likely to have an undesirable outcome if you're looking for your battery to last a while. Qualcomm also mentions though that music is sent to the DSP for "supported audio types such as MP3 and AAC," which leaves and open possibility for some audio formats to still use the CPU for music playback from the sounds of it, which would in turn effect your results to get the same amount of battery life as described in Qualcomm's test.