Sonos "Coupons"

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.

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.