When emerging the package dev-tex/tipa the file /usr/share/texmf/dvips/config/config.ps should be updated, namely the following line should be added to such configuration file: p +tipa.map I had to do this manually. If you don't do it, you will not able to visualize the International Phonetic fonts by using dvips and ps2pdf for Postscript and PDF files.
Added some info to the ebuild to inform users how to use tipe with ps tools. Thanks for reporting.