Using IEEEtran with siunitx's \num{...}

You should use \sisetup command in your preamble in order to tell siunitx detect the font weight.

This can be done by using the command:

\sisetup{detect-weight=true,detect-inline-weight=math}

(as suggested here)

Or—as you found out yourself—the command:

\sisetup{detect-all=true}

Full MWE:

\documentclass[conference]{IEEEtran}
\usepackage{siunitx}
\sisetup{detect-all=true}
\begin{document}
  \begin{abstract}
    foo \num{12.34} 12.34 bar.
  \end{abstract}
  shit
\end{document}

Or

\documentclass[conference]{IEEEtran}
\usepackage[detect-weight]{siunitx}  % <---
\begin{document}
  \begin{abstract}
    foo \num{12.34}\,bar or \SI[mode=text]{12.34}{bar} % <---
  \end{abstract}
\end{document}