The lore around absorbing the soul of a Dragon is a pretty big deal if you really think about it. I wanted something that sounded a bit more ominous, mysterious, and foreboding, so I composed a new piece specifically for this event (I'm a musician.)
It replaces all three variations of the regular music that plays after you absorb a soul. I may make some alternate versions.
To install, just add it with NMM or MO, or extract to your /data/ folder. It's just an audio file replacer, so there are no plugins or fuss. I highly suggest playing with Silent Skyrim - No Music for extra immersion, which mutes regular music but keeps event-based music like this.