Although I know more about data transfer than audio, we have lots of analogue copper lines in the telephone business. A good splice is pretty much completely negligible. According to our best meters, the electrons don't seem to mind at all. In our business the danger is more in that it has introduced a weak point for humidity and eventually corrosion. For this, we use special gel filled connectors to splice copper. John is correct it must be solid. These connectors, if not properly crimped can impede data or voice transfer.
Fiber splicing is a bit more interesting. Even a good splice will show up on a meter but is not necessarily damaging data unless it's really bad. Our meters can visually display the distance to a splice, improper twist or (gasp) a cut.
I suspect it is much the same case for audio equipment. Do a good job, and you should be fine.
