With the help of the given links I was able to solve the problem myself. The correct way is to get the resource ID with
getResources().getIdentifier("FILENAME_WITHOUT_EXTENSION",
"raw", getPackageName());
To get it as a InputStream
InputStream ins = getResources().openRawResource(
getResources().getIdentifier("FILENAME_WITHOUT_EXTENSION",
"raw", getPackageName()));