Roblox can't play a .mid - a Sound plays an uploaded audio asset, so a MIDI has to be rendered to audio first. Drop one here and it's parsed, rendered, and handed back as a WAV, entirely in your browser. Pairs with the MIDI Maker: write it there, convert it here, upload it to Roblox.

What the render sounds like. This uses the same synthesised instrument voices as the MIDI Maker's preview, so a file you wrote there converts to exactly what you heard. A MIDI from somewhere else will play the right notes with the right timing, but through these synth voices rather than real recorded instruments - for orchestral realism you'd want a DAW with a soundfont.
Why WAV, not MP3. Browsers can decode MP3 but not encode it, and this site only loads scripts from its own origin, so MP3 would mean adding a permanent encoder dependency. Roblox accepts .wav directly, so this skips that - at the cost of file size, which is checked against Roblox's 20 MB limit below.