How do you turn on sound for Find my iPhone? Mine only vibrates. Thanks

Click All Devices, then select the device you want to play a sound on. If the missing iPhone, iPad, or iPod touch (iOS12.1.1 or later) is online, a Find My [device] alert appears on the device's screen. If the device is an iPhone, it also vibrates. After a five-second delay, the device plays a sound.

iCloud: Play a sound on a device with Find My iPhone - Apple Support

https://support.apple.com/kb/PH2699?locale=en_US