Musepack (MPC) is a lossy codec tuned for transparency at mid-to-high bitrates. In the early 2000s it was the quality-first choice for listeners who found MP3 lacking, and collections from that era often carry a permanent .mpc layer.
Why iPhone can't play Musepack natively
Apple never supported Musepack, and unlike lossless formats there is no clean escape route: converting MPC to AAC or MP3 is lossy-to-lossy and permanently degrades files that were encoded for quality in the first place. The only way to hear them as encoded is a player that decodes MPC directly.
How BTR AMP plays Musepack
BTR AMP plays Musepack natively — added in version 15.0 with APE, TTA, WavPack, and the OGG codecs. Files play exactly as encoded, with the codec shown per track, and the on-device metadata editor supports APE tags, which MPC files use. Fittingly for the format whose community originated ReplayGain, BTR AMP includes ReplayGain volume normalization, so mixed-era libraries play at an even level. Your MPC files live in one library with everything else the app plays.
Getting your Musepack files onto your iPhone
MPC files transfer like any other format.
Send files over Wi-Fi from any browser on your computer (Connect → Wi-Fi Transfer), pull them from Google Drive, OneDrive, iCloud Drive, an SMB share, or WebDAV, use AirDrop, or play straight from a USB drive. The transfer guide walks through each method, and the importing guide covers the details.