Help! Source Output Too High for Headphones

In-line attenuator?

It's called a resistor.

There is no volume adjustment on the Sony, unlike my Oppo 105D.
Can I put anything in-line to reduce the output?

A Goldpoint SA2 or Schiit SYS will do the trick, you can connect it in reverse if you want to use it to switch between two headphone amp.

https://www.goldpt.com/sa2.html

https://www.schiit.com/products/sys

iFi iEMatch+ 3.5mm Male to Female Headphone Jack in-Ear-Monitor Audio/Optimizer/Attenuator https://www.amazon.com/dp/B08S55MPH5/ref=cm_sw_r_cp_api_glt_fabc_2303E6P2562XQB5EVKYG

+1 on the iFi iEMatch recommended by @dconsmack 

Just thought I'd follow up. I received XLR attenuators for my balanced output, and Schiit SYS for the unbalanced output. Both work great and kept me from buying a preamp.

Thanks for the suggestions!

Yes, there are many products that can cut the volume. Excessive loudness was a common problem with older airplane headphone jacks. I bought a $10 Koss product 1 a long time ago for air travel alone (it’s pretty flimsy and just okay IMO).
With a quality full-size CD player, I’m guessing you’d want something more substantial. There are plenty of simple desktop volume controls of varying quality (example 2), and more complex pro audio items too (which typically use 1/4" or XLR cables; example 2). Beyond this, you could buy a passive or active preamp 1 with volume, balance, tone, source selectors, etc.