Stranded.
Having worked in a metrology lab for an aerospace bearing manufacturer, an R&D department for a major gas &oil company, and in gas & oil operations with electronic controls and monitors protecting millions of dollars worth of equipment, if it's good enough for that duty, it's good enough for my stereo.
Belden, Canare, or other high quality cable.
FWIW, in industrial applications we almost never use solid conductor wire. They have a tendency to fatigue and break at junctions. Especially if your wire strippers put a good nick in the wire when you are working with it. Thermocouples being an exception, but those have been replaced by thermistors and RTDs, for the most part.