IPA in math mode

I suggest you try

$\textit{\textipa{I\textrtails}}$

to obtain the slanted versions of \textipa{I} and \textipa{\textrtails} while in math mode.

\documentclass{article}
\usepackage{tipa,amsmath}
\begin{document}
\textipa{I\textrtails} 
$\textrm{\textipa{I\textrtails}}$
{\itshape \textipa{I\textrtails}}
$\textit{\textipa{I\textrtails}}$
\end{document}