Was it ever explained even remotely how Supeman could use his ventriloquism in space? Was it ever described as potentially being low level telepathy or what?
Some have argued that this is precisely what Super-Ventriloquism is, that it is a side-effect of Superman's advanced intellect, sort of like Super-Hypnotism, taking the form of a type of broadcast telepathy, which accounts for why it can do things that real ventriloquists cannot do, like make their voice seem to come from a different place. Though you're right, it is a mystery.
One POSSIBLE explanation, not telepathic or technological in nature, is based on Superman's vocal powers. Superman has been stated as being able to reach sounds with his voice that only dogs can hear, for instance, and hum at just the right frequency to shatter steel. Is it possible that he can make his vocal chords vibrate into the radio range? If so, the ears of members of the Superman family can detect radio and they in fact, speak in this this way to each other.
DC is right now creating a "Magic Bible" that explains how Magic in the DC Universe will officially work from now on. I wish others could do the same for telepathy and mental powers as well - make sure they all follow consistent rules. In the LENSMAN books, for example, telepathy had a distinctive way of working: for one thing, it was not possible to lie via telepathy. When speaking to another telepath, communication was ten times faster than through speaking, and when a telepath died, he let out a terrible "deathscream" that the minds of other beings can hear.
And what other ways did writers over the years have for the Super family to communicate in space?
I saw them using a type of morse code once... Sign language a few times as well.
The explanation for why the Legion of Super-Heroes can communicate (and survive) in space is because they have transparent transsuits, which open immediately on contact with the vacuum (though they aren't activated by anything else, especially sudden changes; this accounts for why Legionnaires can be overcome by gas attacks), and they communicate in space by "telepathic earplugs." Perhaps Superman uses some version of a telepathic earplug?
When Superman sped faster to intercept light, he had to read lips, and thus cannot hear anything when someone's back is turned to him.
Dilber MC Theme by HarzeM