Southern Rail Models were first on the scene to offer a truly sound ready model. No need to fit a speaker... just plug the decoder in! A 20mm 4 ohm 1 Watt speaker comes pre-installed in the fuel tank area. The speaker is wired back to the 21-MTC plug, so it's a simple matter of installing the decoder once the body has been removed. The installed decoder is a tight fit against the inside bulkhead which houses the exterior fans. The lightboard may need to be unscrewed from the interior and moved 1 or 2mm forward, toward the driver's cab.
Note the decoder orientation! Take care and double check the pins aren't out by one row. If the decoder is mounted incorrectly and powered up, it will most probably fail immediately.