Any new load on a network will degrade it's performance. The questions are how much additional degradation does each new funciton add and how much additional degradation can the other network functions tolerate.
You also did not mention the existing network bandwidth. Ethernet comes in 3 flavors (last time I checked) 10 megabits, 100 megabits and 1 gigabit (there may be a version around 200 gigabits also).
Voice messaging will work with minimal degradation on any version.
Real time video streaming will degrade anything but the 1 gig substantially.
You mentioned a lab. If you are using the LAN to run a real time operations risk of degradation will depend on the real time task frame rate(s) or if the task is asynchronous and how many data packets are required. If your existing tasks are not real time then risk is minimal.
There are limited ability LAN throughput tools available in windows to monitor network load and there are free or trial tools on the web for download.
BTW You did not mention operating system OS for terminals and server. Or is it peer to peer?
Now to the real answer. First you have to create a statement of work:
1) Define existing network, HW & SW and functions performed with the bandwidth required.
2) Define growth requirements in order of priority and estimate their bandwidth requirements.
3) Define special requirements like real time process control.
If all this sounds like work. It is, that's what life turns into after you graduate (plus the proposals, budget requests, power point charts, etc).
Short answer is unless you're running real time functions go download "ventrillo" for free. Install the Host software on the server and a client copy on each PC on the network. It won't even degrade a 10 meg network noticeable since it uses compression and it has excellent sound quality. The software can be configured for VOX or PTT (that's voice activated or push to talk mapped to a normally unused key). The software will allow 6 clients and duplex 2 way communications.
PS I hope I didn't scare you too badly with that real life stuff up above. Enjoy still being in school. It goes down hill fast after graduation. And stay in India. Thats where all the jobs are headed.