Is playback really lossless?
Yes for 16- and 24-bit masters, exactly. 32-bit float is quantised to 24-bit. Delivery carries a SHA-256 checksum either way.
For a 16- or 24-bit master, yes, exactly: Soneam encodes a FLAC preview so it plays in any browser, and it decodes back to the same samples, bit for bit. Nothing is turned into MP3 or AAC anywhere in review, which is not true of every tool that shares audio with clients.
32-bit float is the exception, and worth being precise about. FLAC stores integers, and the preview is written at 24-bit, which is what every browser decodes. A float master is therefore quantised: anything above full scale is clipped, and values below it land on the nearest 24-bit step. If you are working in float and want the client to hear it untouched, deliver 24-bit.
Two claims, and they are about different things. Playback is lossless: what the browser decodes matches the file, subject to the float caveat above. Delivery is checksum-verified (SHA-256): the hash is recorded when the package is made, so anyone can confirm afterwards that what arrived is byte-for-byte what you sent. The second holds for every format, float included, because nothing re-encodes a delivered file.
One more thing playback cannot promise for anybody: the browser hands audio to the operating system, which resamples to whatever rate the output device is running at. A 192 kHz master decodes correctly and then plays through the device your client's machine is set to.
The loudness preview and Fair Loudness both work by attenuation only, so they change the level and nothing else. Turn them off and the signal path is transparent again.
Not covered here? support@soneam.com.