Remember that there are many factors in the “real world” that will affect power consumption, and subsequent battery performance.
Number one on the list is the file type and data rate. The champion for popular quality versus size is the venerable 128kbps mp3 file. Easy on the Clip, decoding is simple, and reasonable audio quality. If you have DRM-enabled data, wma files and higher data rates, the Clip has more processing to do.
Number two is the output load. The “stock” headphones are 16 ohm open type buds. Depending upon the ambient noise level, they have to be turmed up a bit. With an in-ear type, like the JVC Air Cushions, the volume need not be nearly as high, since they are more efficient. I have some big driver Macally headphones that are 32 ohm: less efficient, but higher impedance.
As I feel it to be the equivalent offense to plopping ice cubes in a glass of cabernet (or heaven forbid a single malt!), the Clip is capable of wonderful sound, so I won’t go lower than 160kbps wma, preferring 192 or 256 for imported CDs. I venture into the lower rates for podcasts and books, where the source is speech, not requiring much in the fidelity department, nor in the processing arena. Still, running Rhapsody music at 160/wma/DRM, I get 12 hours, including regular pauses.
Be sure to have the latest version 1.01.29 firmware, as power management has been improved in subsequent FW updates.