Open Settings › CALIBRATE and tap along to the click ten times. Do this again whenever you switch between the speaker, wired headphones and Bluetooth headphones. Bluetooth adds latency; wired headphones or the speaker give the tightest timing.
It's too easy to read the answer from the screen.
Settings › HINTS: LIGHT shows only the call, EARS is sound only. Every level still shows where you pressed and whether you were early or late.
The effects are too intense.
Turn on Settings › REDUCE FLASHING. It is on automatically when iOS Settings › Accessibility › Motion › Dim Flashing Lights is on.
Are the levels locked?
No. All sixteen levels and all four songs are open from the start.