Nope. Because you do not ask any questions, I do not know which puzzle pieces are missing in your knowledge. Please, open up a new thread, ask questions, and then I can explain this to you. If you think you are more clever than me and do not need that, then I am of no help. Chazz needs a TLS connection to negotiate the sRTP keys, and I provided him one way to go. Again, even with an offline computer (and/or a lab scenario) you are able to use a public know certificates even with host-name validation. If he wants to use Let’s Encrypt, the trick is to copy the certificate over and install a custom DNS server on his offline computer. If he is able to turn off host-name validation in his SIP phones, disabling part of the authentication, he even does not need that DNS server. In any case, he gets encryption this way.
Mhm. I guess, this is part of your research then. In short, newer does not matter.
Some starters: sRTP are just encrypted (and optionally authenticated) data packets. The problem is how both communication partners know about the keys used for encryption. For this, a plethora of standards exist, like DTLS-sRTP, SDES-sRTP, and ZRTP-sRTP. Much more standards exist but those three are the ones I see in current SIP phones. Those are part of the bill (processor cycles), when it comes to setup a sRTP connection. For example, SDES-sRTP requires SIP-over-TLS because otherwise the keys for sRTP are transmitted in clear text. DTLS-sRTP requires SIP-over-TLS (not because of the standard but because of the implementations) and requires DTLS. However, this is a one-time, call-setup operation. Depending on your research, this might not be interesting. Depending on your research, this might be crucial.
Therefore, when you researched that, you should contact your supervisor if he wants that part of the bill as well. BlazeStudios mentioned that shortly: With some SIP phones you can do SDES-sRTP without SIP-over-TLS (although this is gives no protection at all). At my link above, you find a video and white paper which tackles this a bit.
I would go into two directions: Try to setup a phone call with sRTP (practical part). My link above should help with that, especially when it comes to configure the clients. On the other hand research the various sRTP key negotiations (theoretical part). When you have done that, sync with your supervisor whether to combine those or concentrate on sRTP.
When you setup a call with sRTP, you have the options your SIP phone provides: SDES-, DTLS-, or ZRTP-sRTP. For the start, I would go for SDES-sRTP. Then you have two options: Go with SIP-over-TLS or not, depends again on your SIP phone. If you go for SIP-over-TLS, you have again two options: Go with authentication or not. The best documented path is to go for authenticated SIP-over-TLS with SDES-sRTP. That works with ‘every’ SIP phone but is the most work to setup.
Again, at my link above, you find a practical tutorial to setup an Asterisk in such a way. Go through that, and if you have questions ask them here, or via private mail.