Power Mixer allows you to take better control over the audio configuration on your system. It replaces (but doesn't disable) the internal Windows volume control, which has never worked great for me. There are a couple of things that Power Mixer does that I find very useful. First, it has a concept of "presets". A preset allows you to set the volumes for all of the audio devices in your system all at once. This may sound a little strange for people that only have one audio device, but I imagine many people have more than one audio device on they system and don't even know it.
Video cards now even supply audio devices to the system via HDMI, so that you can send audio to your monitor, if it supports it. I have two such video cards in my PC, as well as an on-board audio card, a USB headset, and webcams that support microphone input. I can use Poewr Mixer's preset feature to set specific volumes for all of the devices all at once. I can also use it to set the default output device, which is very handy, because it allows me to easily switch between my office speakers, headset, and bluetooth speakers in the kitchen.
Presets can be managed via hotkey, so I can press Windows+Ctrl+S to get my office speakers, and Windows+Ctrl+D (these were programmed by me, and are complicated only because practically all other combinations are in use already) to get my headset. The headset volume is pre-set to 30%, which is much better than the 100% required to drive my office speaker amp. This is very useful.
BlueSoleil is a crappy little junk application, and it's not the sort of thing I would run on my PC, except that it has a feature that I can't find anywhere else: It advertises the computer's services via bluetooth.
Normally, if you have a bluetooth headset, you put it in pairing mode, and your phone (for example) can find it and pair with it to send audio to it. Most of the time, your computer consumes the services of bluetooth devices this way. Your computer will use a bluetooth mouse or send audio to a bluetooth headset. What I wanted to do was connect to my PC from my phone as if my PC was the headset.
Normally when you pair an iPhone (what I have) to a PC, the PC connects to it using the Personal Area Network service, and uses the iPhone as an internet gateway. This is handy when there's a Verizon network disaster and I need to connect my PC to the internet, but that doesn't really happen. It's more useful for me for the connection to go the other way, so that I can send audio from my phone to the PC.
Mostly I use this to listen to audio books. Rather than use the clunky Audible client on the PC, I just download the books to my phone, and use bluetooth to broadcast them to my PC, where the audio is played out of my office speakers. This is quite handy. This is what the BlueSoleil software allows.