The Spectrum Next can create sound using Turbo Sound Next (3 AY-3-8912 sound chips) controlled by the BASIC PLAY keyword and the signal from these goes to the HDMI port or the 'ear' socket. Additionally an internal speaker can be connected which will generate sound in the same manner as the original 48k Spectrum, controlled via the BEEP keyword, which will also output via HDMI/ear.
A speaker attaches to connector J3 on the circuit board and requires a 2.54mm Pitch, 4 Way, 1 Row through-hole pin header. If you're fitting the board inside an original rubber key case, you'll probably need a right-angled connector due to the height required.
The buzzer should be approximately 1 inch diameter with impedance of 16 ohm (which should equate to a basic 'PC BIOS' piezo type speaker). J3 pins 1 and 2 are negative, pins 3 and 4 are positive.
The internal speaker can be disabled from the TBBlue boot ROM options menu.