1. Modern VoIP applications are incompatible with Windows 95/98
2. Hardware availability
3. Networking issues
4. Driver support
5. Obsolete Protocols
6. Performance limitations (indeed, modern audio codecs may be an issue, incl. Opus and AAC)
A 250 MHz processor can handle basic voice chat but with significant limitations, and there is much more to it when it comes to practice... so while technically a 250 MHz processor might (for historical experimentation or nostalgia), in reality it is not practical for functional use today.
A Pentium III 733MHz with 128MB RAM running Win98 works fine for (low-res) video chat. Been there, done that.
No they aren't. DVD playback on early systems was hardware based using an MPEG2 decoder (often included on the sound card that came packed in with the DVD drive, or on some video cards or dedicated MPEG decoder cards)
Software playback of DVD's on a Pentium MMX at 233mhz is going to be limited to single digit framerates, especially if you are trying to decode Dolby Digital or DTS audio as well
I would be more concerned about modern video codecs. None of them are exponential time, but they need so much compute that it is unlikely that a Pentium II could handle them.
https://en.wikipedia.org/wiki/Microsoft_NetMeeting
NetMeeting even used an open ITU protocol that could voice call other clients! The 90s were a different time.