Are your CD connections on a DIN plug or RCA Phono connectors?? I'm not sure there are phono "Aux" connections, in which case you can try "Tape 2" In.
If the sound seems to be mostly there but your are missing the "centre stage" can you check the polarity of your connections from the CD player - it is possible that if your have one channel with reverse polarity compared to the other you can end up with a "hollow"sound stage (note that vocals are invariably panned dead centre).
Generally speaking, you aren't learning much if your lips are moving.