The young man is telling you the truth.
Even if the cable box passed audio vis HDMI, it's sound gets no beter than Dolby Digital, which is passed by the optical digital link you currently have.
And, unless your receiver is capable of decoding the new hi-rez audio formats available with bou-ray players, and I doubt it is, you're doing a good as you can get.
Theoretically, You might be able to get a wee bit better sound
if your Blu-ray player can internally decode them
and pass them through six analog cables to your receiver
if it had the needed six analog inputs to accept them.
But, on a more practical note, your receiver won't allow that, so you're set up about as good as you can hope for. The young man is telling you the truth.
Your HT-SS2300 :
http://electronics.pricegrabber.com/home-theater-systems/m/83945703/details/st=product_tab/