How to use iPhone to send Bluetooth to a speaker playing the music from my PC HDD?
You can't use your iPhone as a receiver of music (either from wifi or cable) from your PC in the manner you appear to indicate.

However, if you have iTunes installed on the PC, you can use iTunes to access your music first.  Then, use the iPhone to access the music through iTunes.  Once you've got the music playing, you can output it to your BT speaker.  It's actually much simpler than it looks in print!

You would need to setup your devices to use Home Sharing.  Start by following the guide in this Apple Support page on Home Sharing.  All devices must be on the same network for this to work.

Here's another walk-through on how to set this up and it might be a little easier to follow.

The benefit of this setup is when you add more devices, like Apple TV or an iPad, they can also access this same music using Home Sharing.