Salsa20 is a stream cipher. DTLS and SRTP are higher level security protocols that use ciphers (among other things) as building blocks, to ensure things like replay protection, integrity protection, mutual authentication, secure session key agreement and forward secrecy in addition to confidentiality.

If you replace an engineered security protocol with a raw cipher and key, you create many vulnerabilities and make a much less secure system. VOIP applications are especially vulnerable to replay attacks, for instance.

A better approach would be to continue using DTLS and SRTP, and use Salsa20 as the SRTP cipher.

Non-crypto-experts saying things like "we replaced <estabilished security protocol> with <my own idea> and it's much faster" is a well known bad sign - It often indicates the person has fumbled things without realizing it.