The loudspeaker is essential for a good sound. Basically, the speaker should be as big as possible - easily said, but difficult to achieve due to the limited space in a scale model.
It's important to use the proper impedance - expressed in Ohms. You can always take a higher Ohm-type than recommended. The other way around, however, is critical: If using 4 Ohms instead of 8 Ohms, the power amplifier could be overloaded and the sound module could get hot.
You can use a different speaker than recommended, but please mind the correct impedanceand always take a broadband or voice application type. Due to their high-frequency suppression characteristc, bass-type speakers are not suitable. Turbo sound and reversing beeper will almost be completely inaudible with these types of speakers. Power rating should be at least 4 Watts, but can be higher as well.