How to replace wire next to another wire?
I would approach it first by trying to pull the network cable through with one of the audio wires.   Strip the covering from about 6 inches/10cm of the network cable and separate the conductors in the audio cable about the same distance.  Put the two cables end-to-end with the stripped parts overlapping and fold half of each cable back around the other and use electrical table to tape down flat to the original cable (folding the full cable may make your joint thicker and more likely to catch on the other wires in the conduit).   Tape the remaining conductors down flat to the opposite cable (I adjust how I do this for the flattest seam with the specific cable being pulled).     Now, see if you call pull that one cable through.   If not, you may need to pull all three out, in which case I would pull a "pull string" rather than a fish tape -- electricians use a synthetic string that is lightweight but slippery -- and then pull the wires you do want back through.

If you have access to the attic, you should check to see if the conduit is continuous or if you will need to make one pull into the attic and a separate one back down.

Depending on the length of the run, you may also want to look into wire lube that eases the effort to pull them through the conduit.