Firstly, you don't need to send them back the old product. You can keep it. I have done exactly that. 

Secondly, was it not clear at the checkout what price you were paying?

You clearly didn’t read the Terms and Conditions of the Upgrade Program.

It is common practice that discounts cannot be stacked.

There is also no requirement to return the device to Sonos when using the upgrade credit.

We couldn’t get the Play 5 to work after multiple troubleshooting attempts with customer service. It didn’t work, and they offered a 30% off coupon if I sent it back to them. Didn’t want a big paperweight, so took them up on their offer. While shopping Sonos.com, I noticed the speaker replacement we wanted was already on sale, so my voucher wouldn’t work. Customer service says they are unable to stack vouchers with items already on sale. Never mentioned that to me before. They are surfing in shady legal ground.

Always been "only one discount" pretty common practice most places.

Was it a Play:5 (Gen1) or a Play:5 (Gen2) ?

Gen 1. It was out of warranty, so the offer seemed fair.

But this would have been evident at the checkout, right?

You must have seen the cost when you entered your payment details. 

Never made the transaction. Their system wouldn’t accept the voucher they gave me. Said since it was  already on sale, they wouldn’t honor the voucher. Like I said very shady business practice, since they never informed me of that prior to sending my old Play 5 to them.

The trade-in program gives you a discount that will apply at a time when speakers are not on sale. Use the sale price for a purchase now, and the discount voucher at a later time if you want to add another speaker. 

The voucher was offered over the phone. Legally, it should have been disclosed beforehand that the offer was only good on full priced merch. 

I have no legal training, so cannot comment on that statement.  But if full disclosure is required, as you imply, should they also have gone through the Terms and Conditions of Sale, the User Licence Agreement and so forth as well? They may have changed since your last purchase. 

Sorry dude, but you're in the minority on this one. People aren't shy to put the boot in on Sonos either in these parts. 

Are you suggesting that the entire user agreement should have been read to you during the call? 

​@Chicagogator 

How many times have you bought something online at a reduced price and tried to use a discount, only to find that the discount doesn’t apply to sale items. This cannot be the first time you’ve come across this. Nothing shady about it, it’s entirely commonplace.

In your case, not only can you buy a replacement in the sale, but you are also left with a 30% off voucher for something else. It’s not all bad.

If it was a play 5 gen 1, then you would have a 30%  disocunt offer regardless of the condition of your existing speaker, and no need to ship it back.  Sounds like there may have been a miscommunication with customer service.

From my experience, sonos.com gives you the option of using the discount off full priced item, or take the sale price, saving your discount for later.  Indeed that seems to still be the case. 

I don’t see anything shady here.  Unless you were told by Sonos that discounts would stack, there is no reason to assume they would.

When it goes down like this, how would you feel….

My product isn’t working. Troubleshoot with Sonos. They offer OVER THE PHONE to give me a 30% off voucher for a new product at Sonos if I send back my defective out of warranty speaker. PERIOD!

I agree and they send me a postage-paid UPS label. I send them my Play 5. Get the confirmation they received it and the coupon code. I go to Sonos and find the product. Offer is no good. So what if it's on sale.

Again, practices like this are deceptive and a horrible way to approach customer service.

Support won’t necessarily know of any sales/promotions that may be running. I’m sure the voucher was offered with the best of intentions.
 

Are you expecting 30% off the 30%-reduced price or a 60% discount? 

Perhaps it would have been better if they simply said “Your product is many years old, and is well out of its warranty period. Sadly, electronic devices do fail, sooner or later. You are a victim of such a failure. Sorry, but there’s nothing we can do.”