Migrate legacy C code

I need to convert this legacy C module used across our engine to idiomatic C++ so we can manage ownership and avoid leaks. Take the source files in src/legacy_audio.c and src/mixer.c, replace manual malloc/free with RAII—use std::unique_ptr or std::vector where sensible—turn struct-like APIs into small classes with clear private state, and keep the public functions compatible for now. Before finalizing, run the existing unit tests and point out any ABI-breaking changes.

Before I hand this to the game systems lead, make the migration easy to review and accept. Move each resource allocation into a constructor, expose a clear destructor, show a one-page diff that surfaces ownership changes and newly added noexcept or deleted raw frees, highlight places where behavior might change (pointer aliasing, thread safety) and propose a compatibility shim when needed.

I just replaced malloc/free in mixer.c with smart pointers and the tests still pass, but valgrind reports retained blocks from the mixer subsystem. I am unsure whether leftover references are from external callers still holding raw pointers or from cycles created by callbacks. The lead expects zero leaks before the branch merges. What is the most likely cause and the minimal next steps to find and fix it?

Over the last three projects I convert similar C subsystems and we repeatedly get ownership regressions and late leak fixes during QA. I lose time debugging, and reviewers complain about mixed styles. Which habit should I adopt across all migrations to stop rework—how to structure commits, tests, and a migration checklist so ownership issues are caught early and reviewers trust the changes?