Hey man, newbie here, too...but have been living with this stuff for many many years.
So, here is your issue: your deck wants to see 1.5V as an input level.
Your MP3 player makes about .05V (yes, that is NOT a typo, about .05V) so the chance of getting happy levels is slim. The CD input should be a bunch closer to the Sonys output level, but I would bet it is not gonna do well, either.
If you use a decent quality "Head Amp" that will fix the issue. There are none left on the market like the old Zapco piece, or even the Phoenix Gold (both of which retailed for about $200 BITD) but PAC makes the Turbo 1 which will do the job nicely.