To begin with, it must be noted that comms are a plot device, which means that they will work any time the authors need them to work in order to advance the story, and they will not work any time the authors need them to not work in order to advance the story. Anyway...
1. Video, sound and text pretty much cover what comm systems should anyway, so I don't entirely understand your question
. Presumably, comm systems are capable of transmitting anything that can be transmitted - though, not knowing whether these are digital or analogue transmissions, we don't know their exact capabilities.
2. According to the WC1 manual, (some?) Kilrathi fighters have a comm system that only has a range of 5,000 metres (or 15,000... I can't remember). But even this limitation is inconsitently applied. Blair in WC1 can be taunted by any Kilrathi ship, even one that theoretically is beyond comm range. Confed pilots are always able to communicate with their base ship when the storyline requires them to - no range has ever been defined.
3. We don't know - in WC2, the Concordia receives a transmission from a different system, but it's not explained how this works. The WCP manual talks about some kind of transmission system called FLASH (note: FLASH may not be the name of the system - in may be an acronym signifying the contents of the message, kinda like SOS), but no explanation is given as to how that works, either. The book Action Stations also includes a special type of comm system (translight comms, IIRC), which apparently provides near-instantaneous transmission even on such distances as McAuliffe to Earth - but of course, it never works because the book's storyline doesn't allow it to
. It may be that WCP's FLASH system is somehow related to this, and it may be that this is what all those comm relay stations in WC4/WCP are there for (in any case, WCP does imply that the Midway cannot communicate too far without the Hhrass Relay Station... but since the Midway is not in Hhrass when this problem is noticed, we can conclude that the Midway can transmit from one system to another).
Oh, the WCM movie also deserves a mention here - apparently, in 2654, the fastest way to get a message from Tolwyn (somewhere in Enigma Sector?) to the Tiger's Claw (in Enyo, Vega) is to send a drone to Pegasus (Dakota, Vega). It's not entirely clear why drones are used for comms in 2654 instead of those translight messages from 2634 - the real answer, of course, is that the writer of the movie script most likely never read AS, but I don't know what sort of in-universe explanation would best explain this.
4. See above.
5. Depends. Some transmissions are real-time (in-system ones always seem to be), while others can have a significant time-lag. In 2673, it takes a few days (weeks? Again, I can't remember the details) for a transmission of the senate debate (WC4 intro) to get to Blair on Nephele. However, in 2681, Intel on Earth is aware of what's going on in Kilrah almost at the same time as it happens. There are two possibilities here - a) there is a significant difference in the quality of civilian and military comms, with the latter working almost real-time while the former are very laggy, and b) the time lags of 2673 would no longer happen in 2681, which seems reasonable if we assume that the Hhrass Relay Station and other similar bases were built in the 2670s. In UE, I've assumed that the time lags do still exist beyond a certain range - but of course, UE cannot really be used to prove anything.
6. Radar, no - comms and radar are entirely unrelated. However, comm silence would prevent enemies beyond radar range from detecting you. Also, comm silence is not always necessary to remain undetected - one or two WC sources talk about laser-link comms, where messages are sent along a single beam to the receiver. In such a situation, the only way messages can be detected is if somebody gets directly in the path of the beam.
7. No idea.
Note that I probably got a few things wrong above, since I'm working almost entirely from memory.
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