Otherwise, if you want to smack proper vectors and matrices together at high speed, libraries like Eigen or DXMath already abstract away the SIMD details and work great. For nitty-gritty stuff like codecs, that's always going to be handwritten with intrinsics (or ASM), and that's fine. And libc functions like memcpy already use the fastest, fanciest instructions. It's mostly a solved problem.
Lastly, for a lot of tasks, regular math instructions are plenty fast. On modern CPUs, you need to be doing a lot of math before worrying about SIMD is worth it. And once your program becomes particularly math-heavy, you'll probably want to use the GPU instead anyways.