But it is the same as the source - the source is a 32-bit Floating Point mix. As I said before, the format of the files you used in the first place is irrelevant. If you want to export your mix as a 24-bit file, then you have to click on the Sample Type's 'Change' button and select 24-bit there. At which point your mix file will be exported as a 24-bit integer file.