Compression waves traveling at less than the speed of sound can still be considered sound waves. Sound waves are simply compression waves that propagate through a medium, typically air, water, or solids. The speed of sound in a medium depends on various factors, such as the properties of the medium and its temperature.
When a vibrating membrane produces compression waves, it creates variations in air pressure that travel through the surrounding medium. These pressure variations, even if their maximum velocity is less than the speed of sound, still qualify as sound waves. The sound waves may have a lower intensity or volume compared to waves generated by a source that produces greater pressure variations, but they are still considered sound.
The volume or loudness of a sound is determined by the amplitude of the sound waves, which corresponds to the maximum displacement of the vibrating particles in the medium. So, even if the maximum velocity of the vibrating membrane is lower, it can still produce sound, albeit with a lower intensity.