Most PCs have line-level stereo audio inputs and outputs. Their signals are compatible with your receiver and BD player, but you may need an adapter. It would connect between your PC's stereo audio connections (usually a small diameter pin-jack) and the RCA style red/white cables that are used by home audio equipment. Adapters and cables are readily available at stores like Radio Shack and Best Buy.
Most Blu-ray players have red/white stereo analog audio output connections. You can connect those directly (using an adapter) to your PC's stereo audio input connection, or you can cable them to an input on your receiver and cable one of the receiver's stereo red/white output connections to the PC.
You'll only be able to record stereo audio that is connected to your receiver using red/white connections. You can't record audio coming to it by either HDMI or digital audio connections. (i.e. stereo analog audio is upconverted to digital or HDMI, but not downconverted from them to stereo analog.)
You may need to get appropriate PC software for recording its audio input in MP3 format, but some probably was included with your PC.
