You can use the optional argument to provide any listings options:
\lstinputlisting[basicstyle=\ttfamily\scriptsize,language=somelang]{filename}
For multiple listings you can set this settings globally (or locally to the current group) using \lstset{<options>}. See the listings manual for more details.
https://en.wikibooks.org/wiki/LaTeX/Source_Code_Listings
From the tagging, it seems that you are using KOMAscript and the class scrbook. Then you change the font either with \setkomafont or \addtokomafont (see section 3.6, page 51 in the revised manual), and with the modifier chapterprefix. Try:
\setkomafont{chapterprefix}{\rmfamily\Large\bfseries}
or, to change to a serif typeface only:
\addtokomafont{chapterprefix}{\rmfamily}
To change all sectioning to the default Roman font, reverse all font changes you have done to chapter, and try
\addtokomafont{sectioning}{\rmfamily}
It seems from your comment that somebody has tampered with the definitions in your classfile. This minimal example works for me, giving me sans serif in heading and body text.
\documentclass[twoside,12pt,a4paper,pointlessnumbers,headsepline,idxtotoc]{scrbook}
\usepackage{fixltx2e}
\usepackage[UKenglish]{babel}
\usepackage{tgheros}
\renewcommand*\familydefault{\sfdefault}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{blindtext}
\begin{document}
\blinddocument
\end{document}
In your document, comment out (or delete) all usepackage{secsty} etc. that changes the headings. Or add packages and commands from you preamble to my code.
If you are going to use Helvetica as sans serif font, I suggest using the Tex-Gyre version instead, by loading the package tgheros. I have changed my MWE correspondingly.
Best Answer
I would try
Replace
\refnameby\bibnameif your class isbook.If you are using
biblatex, the relevant commands are described in section 3.5.7 of the manual (seetexdoc biblatex).