A playback problem can come from the audio output, loading the piano sound, or the MIDI file itself. Change one thing at a time so you can tell what helped. Use the short built-in piece Melody, Op. 68 No. 1, linked below, as a comparison.
Open Album for the Young Op.68 No.1 "Melody" to try this
Operator and editorial approach
The notes move, but there is no sound
Pause, then press Play yourself. A browser may restrict starting audio until you interact with the page. Next check the player volume, the device’s media volume and whether the tab is muted. Also check that sound is not being routed to Bluetooth headphones or an external monitor.
If it is still silent, try the built-in Melody. If that also fails, your own MIDI is less likely to be the sole cause. The first use needs a network connection to fetch the piano sound; check connectivity and wait for loading to finish before trying Play again.
The piece will not finish loading
If a built-in piece fails too, reload the page and compare on another network. An extension can block requests for the piano sound; comparing in another browser without extensions helps isolate that possibility. Do not start by clearing browser storage: that can also erase your practice records.
If only your own file fails, check that it is a Standard MIDI File with a .mid or .midi extension. Renaming an MP3 or PDF does not convert it. For a file exported from notation software, export a short piano passage as a new MIDI and compare. You can do these checks locally without sending the file to the operator.
The sound lags, or the animation stutters
Bluetooth audio can add delay. Compare with the device speaker or wired headphones first. Note Speed changes how quickly notes travel on screen; it does not correct audio-output latency. Change playback speed if you want the music itself to run more slowly.
For a stuttering display, close other demanding tabs or apps and compare with a short built-in piece. If only a large MIDI struggles, its note or track count may be contributing. If the problem follows switching tabs or waking the device, pause and start playback again.
Sound works, but the score or MIDI input does not
Audio playback and score availability are separate. Score view is available for built-in pieces with score data. Use falling notes for your own MIDI or a piece without a score. MIDI keyboard input also needs browser support and device permission; a browser that plays audio does not necessarily support keyboard input.
If the issue remains, use the Contact page and include the piece URL, device and OS, browser, any displayed error, and whether a built-in piece fails too. Avoid attaching personal information or files you cannot share in a public mention. The symptoms and comparison results already help narrow the cause.