There is one already, go into the TTT settings(F1) and check the box "Play sound cue when a round begins or ends". Then go to game setting and UNcheck "Silence the audio while the game window is in the background"
I think this would be better to have a global chat message inform the round had started. Due to people who will KOS people in chat before the round starts, some will just assume and kill the person, when staff is online it's an easy check, but when staff aren't, then there's really little to nothing that people can do. Even if they want to report it, it'd be very difficult to prove it with just screenshots, unless they record, but people don't really record. Some do and most dont
After tweaking with the settings i have found that that round noice only works if you have the game unmuted globally. Thing is, i do like to tab out to give my ears a break from the voice chat, and unmuting makes everything go through. Id much rather a specific noice that works even if i have the other sounds muted, and i know it can happen, ive seen it in other servers.