[Feature Request] RetroAchievements - Rare Achievement Sound

I saw a Github thread about Flycast RA integration and noticed something. The RA developer made a sound called rareunlock - it triggers when the achievement unlocks rate is or below 5%. This would be AMAZING for RetroArch!

Adding a rare achievement sound to RetroArch would provide several meaningful benefits, particularly for players who use RetroAchievements integration.

• 1. Stronger Sense of Reward
     A distinct sound for rare achievements would create a heightened sense of accomplishment. When a player earns something uncommon—especially after significant effort—the audio cue reinforces that the moment is special. This mirrors the satisfying feedback systems seen on platforms like Xbox and PlayStation Network, where differentiated sounds make milestones feel more impactful.
• 2. Clear Differentiation Between Achievement Types
     Standard achievements and rare achievements represent different levels of difficulty or completion rates. A unique sound would instantly communicate that the unlocked achievement is uncommon, without requiring the player to read the on-screen description. This adds clarity and improves overall user experience.
• 3. Increased Motivation for Completionists
     For completion-oriented players, rare achievements are often the most challenging and meaningful goals. An exclusive sound effect would act as positive reinforcement, encouraging players to pursue harder objectives and remain engaged with a game longer.
• 4. Enhanced Immersion
     Audio feedback plays a significant role in immersion. A rare achievement sound—designed to feel distinct yet cohesive with RetroArch’s interface—would make the experience feel more polished and intentional, rather than identical across all unlock types.
• 5. Quality-of-Life Improvement
     This feature would be a relatively small addition technically, but it would significantly elevate the perceived polish of the achievement system. Even offering it as an optional toggle would allow users to customize their experience without affecting those who prefer the current behavior.

Start with the referenced Flycast pull request #2258 and compare its rareunlock behavior with RetroArch’s existing RetroAchievements unlock path. No RetroArch files or tests are identified in the issue, so first locate that integration and determine how the <=5% threshold, sound asset, and optional toggle would fit; done means the rare sound plays only for qualifying unlocks without changing standard behavior.