You will need to edit the article function in apj.bst and provide an instruction to print the contents of the entry's title field. I suggest you do so as follows:
Make a copy of the file apj.bst; call the copy, say, myapj.bst.
Open the file myapj.bst in a text editor and search for the function called article. I believe it starts right after the line that says %%%%%%% End of functions from astrobib.
In the function article, locate the lines
date.block
crossref missing$
Insert the line
format.title "title" output.check
between these two lines.
Save the file myapj.bst, either in the directory where your main .tex file is located or in a directory that's searched by BibTeX. If you choose the latter method, you should also update the TeX distribution's filename database.
Start using the new bibliography style with the instruction \bibliographystyle{myapj}. When switching from one bibliography style to another, it's important to run latex, bibtex, and latex twice more so that all changes are fully propagated.
If you're interested in learning more about the BibTeX infix syntax, I heartily recommend you read the document Tame the BeaST -- The B to X of BibTeX by Nicolas Markey.
The apacite package contains a LaTeX macro that determines the default formatting of the journal, volume, number, and pages fields for entries of type @article. This macro is called \APACjournalVolNumPages, and its default definition may be found in apacite.sty starting at around line 1275.
To modify this macro, I suggest loading the etoolbox package and using that package's \patchcmd macro. Specifically, you should insert the following lines of code in the preamble, after having the loaded the apacite package:
\usepackage{etoolbox}
\makeatletter
\patchcmd{\APACjournalVolNumPages}%
{\unskip, \Bem{#2}}%
{\unskip, #2}{}{}
\makeatother
The macro \Bem is an alias for \emph. Basically, the modification of the \APACjournalVolNumPages macro consists in removing the italics emphasis of the second argument -- the contents of the volume field.
A full MWE:
\RequirePackage{filecontents}
\begin{filecontents}{mybib.bib}
@Article{Pressair,
author = {Press, F. and Ewing, M.},
title = {Theory of aircoupled flexural waves},
journal = {Journal of Applied Physics},
volume = {22},
number = {7},
year = {1951},
pages = {892--899},
}
\end{filecontents}
\documentclass[12pt,a4paper]{article}
\usepackage[margin=2cm]{geometry}
\usepackage[utf8]{inputenc}
\usepackage{amsmath,amsfonts,amssymb}
\usepackage{graphicx}
\usepackage[natbibapa]{apacite} % load "apacite" with option "natbibapa"
\bibliographystyle{apacite} % specify the bibliography style
\usepackage{etoolbox}
\makeatletter
\patchcmd{\APACjournalVolNumPages}%
{\unskip, \Bem{#2}}%
{\unskip, #2}{}{}
\makeatother
\AtBeginDocument{%
\renewcommand{\BBAA}{and}
\renewcommand{\BBAB}{and}
\renewcommand{\BAnd}{and}}
\begin{document}
\citep{Pressair}
\bibliography{mybib}
\end{document}
Best Answer
Ok I found the issue. In the entry (and in the second and some more) I had a _ inside. I had to replace the underscore with a valid symbol (_).