Can I use coaxial cable for audio over 30 feet?
RCA audio cables are normally coaxial in construction, and are fine out to 50 feet or more, normally. I've run 100+ feet when needed for auditorium A/V setups. While there are fancier ways to go long distances, with modern equipment and cables there's little need for them in most cases. 

Most coaxial cable is not particularly suited to use as "output, without amplifier" - ie, speaker cable use. Use speaker cable for that job, it's what it's made for.