From the top of my head, but besides the suggestion by @Brash, perhaps you could do fixed DHCP assignments on the 36 Sonos speakers and then decrease the DHCP scope to only contain those 36 addresses, and set DHCP required on the SSID.
In theory, the Sonos speakers will get their DHCP address and all is well, and non-Sonos speakers will attempt to get an IP address, but since the scope is exhausted, they will fail to get an IP address. Those clients that has statically assigned IP addresses will not be allowed to Associate to the SSID, due to DHCP Required setting.
I'm just spitballing here, so no guarentee that it would work. 🙂
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