Re: That's about Mach 49
Mach number is relative to local speed of sound so a velocity doesn't correspond to any specific Mach number unless you know what the local speed of sound is. For ideal gas the local speed of sound is a function of temperature (and temperature only). As for the temperature of the so-close-to-vacuum-as-makes-no-difference environment of the Voyager probes (assuming analyzing it as ideal gas even makes sense)... I have no idea.