Your mislaid iPad will only play a sound when the command to do so is received by the iPad - which requires the iPad to have an active WiFi connection.
While perhaps counterintuitive, your iPad may still be detectable by nearby devices while the iPad itself does not have a network connection...
Apple devices use their Bluetooth radio to periodically transmit a "beacon" signal that can be detected by other Apple devices that are participating in Apple's Find My Network; devices that detect the beacon will anonymously report detection of the beacon back to Apple's servers - making the detection and approximate location visible to the owner of the missing device. As such, a missing device may be detected even though it cannot receive remote commands.
Unlike many models of iPhone that incorporate an ultra-Wideband (UWB) radio that can aid close-range location, iPad does not benefit from being directly locatable at close range. As such, you may need to resort to a physical search.