Hmm. Looks like you’ll have to transcode your audiobooks. Something like this should do the trick (in bash, at least):
cd /where/ever/your/audiobooks/are
for f in *.spx; do outfile=echo $f|sed -e 's/spx/ogg/'; (speexdec $f - | oggenc -o $outfile -); done
Assuming ‘.spx’ is the extension of your Speex files. I’ve never come across files in that format. Until now, I thought Speex was only used in realtime applications, i.e. VoIP and things like that. 
