Does Solder selection affect signal quality?
I work in both the Pro-Audio and audiophile industry.  As long as the solder is used properly, and makes a solid connection, then the type of solder makes zero impact in audio quality.

The flux inside the solder, on the other hand, can make a big impact.  But here too, if used properly, it won't matter.  Basically, clean off the flux that is supposed to be cleaned off.  Some flux is fine if not cleaned up.  Rosin and "no clean" flux is fine left on there (mostly).