How important is the quality of the solder?
The quality of the solder and especially the workmanship of the soldered spots should not be underestimated for the transmission of audio signals. Bad solder or a soiled solder bath can cause the transmission resistances to increase. This is not only physically detectable, but in rare cases it can also be heard. So, use high quality solder to work with (partially with silver), make sure to provide a clean working environment and do not touch the wire insulation with the solder tip.