B2Gaudio: Turn every phone in the room into one synced speaker

Hey Product Hunt 👋 I built B2Gaudio because of a road trip. Four of us, two cars, one aux cable, and a bluetooth speaker at 20%. We wanted everyone hearing the same song at the same time and there was just no good way to do it — bluetooth multipoint is a mess, AirPlay needs an Apple TV or a HomePod on the receiving end, and the party-speaker apps I tried either wanted an account or drifted so far out of sync within a minute that it sounded like a canon round. So B2Gaudio does the boring, hard version properly: 📱 One phone hosts. Everyone else taps join. No accounts, no signup, no pairing codes, no cloud. Devices find each other on WiFi — and when there's no network at all, they fall back to Apple's peer-to-peer radio, so it works in a car or a park. 🎵 It's not just music. Local files, Apple Music, podcasts, public-domain audiobooks, and local video — so a few people can genuinely watch something together, not just listen. 🎯 The whole product is the sync. There's no Bluetooth anywhere in it. Devices run an NTP-style clock handshake with round-trip outlier rejection so they agree on a shared monotonic clock, the host broadcasts "start this track at instant T", and each listener converts T into its own clock and schedules against it. From there a drift corrector nudges playback rate by fractions of a percent on local files, so it corrects inaudibly instead of glitch-seeking. Apple Music is the harder case — MusicKit exposes no playback-rate API at all — so that path uses a periodic re-sync seek that deliberately aims half the expected drift ahead, letting the error cross through zero instead of ramping one direction all window. 🔒 Free, and actually free. No subscription, no IAP. There's one banner ad and it's non-personalized — I stripped App Tracking Transparency out entirely because I'd rather not track you than ask permission to. What it doesn't do: iOS only (iOS 17+). Same network or peer-to-peer only, so this isn't for listening along with someone in another city. Spotify support is written but switched off, because their API needs the developer account to hold Premium and mine doesn't. I'd love feedback from anyone who tries it with 3+ phones — that's where the interesting failure modes live and it's the hardest thing to test alone. Happy to go as deep as you want on the clock-sync internals.