Hoping to help anyone else that encountered similar troubles installing on OS X. The first issue I encountered was that the package fftw, on which seewave depends, wasn't installing. The fix for this is to install pkg-source (
http://pkgconfig.freedesktop.org/). Then seewave itself didn't want to install, the solution for which was to first install libsndfile, which in turn had an error installing. There is a post
here detailing the error and an answer detailing a complicated fix, but also another answer saying to just use homebrew, which is what I did. Finally, I was able to do install.packages('seewave',type='source') in R successfully.