If it's a actual dealer or shop, defective items would be exempt from the "no returns" clause.
If a individual,,and listed as new, again, defective items exempt from the "no returns".
And, manufacturers will honor online dealers purchased items warranty repairs/replacements. (Receipt?)
Not to be critical, but, have you contacted anyone regarding a return for replacement yet? Actually explain to eBay, dealer, Walthers what is going on, and full details. (Receipt? Does it list a business name, or person? If receipt lists a business name, warranty replacement.)
If it was listed as used, by an individual, and cheap, replacing the speaker is also cheap, and easy, and might just fix the problem.
Also, did you try removing the body shell and rechecking sound level? (I am sure it will be the same.) You might notice a clear issue (loose/pinched wire, debris, etc...) that you can fix in 5 minutes or less, and have a good running, and good sounding, locomotive, with no fuss/mess about anything else, or more money being spent, even occurring.
HO scale Proto-freelancer.
My Railroad rules:
1: It's my railroad, my rules.
2: It's for having fun and enjoyment.
3: Any objections, consult above rules.