This is a quick list of things to try if tracking through phone sensors using the FreePIE app doesn't work:

• See whether your phone model has all of gyro, mag, and acc. You can use the CPU-Z app and uninstall it afterward.
• When mounting on headphones, position as far away from each magnet as possible. Try different configurations.
• The rotation is received through the system's API for position. The only room for improvement is phone hardware's sensor precision.
• There's a working freepie app as .apk inside opentrack's contrib directory inside the installation directory.