In Other Words (A Pretentious Treatise On Loudspeaker Technology).

Loudspeaker efficiency is inversely proportional to its moving mass and directly proportional to the square of the product of cone area and to the product of magnet field strength within the voice coil air gap and the length of wire immersed within the magnetic field

In other words, itty bitty speakers with weak magnets are inefficient.

The maximum sound pressure of a loudspeaker is a function of the volume of air displaced, or placed in motion, by the loudspeaker at any given pitch and also of its acoustic impedance. The volume of air the loudspeaker must displace is directly proportional to wavelength; longer wavelengths, producing gravely-pitched sounds, require greater displacement than do the shorter wavelengths that produce higher pitch sounds.

Constraining the emanation of the loudspeaker diaphragm by means of a horn projector causes the diaphragm to place an expanding column of air into motion, increasing displacement for a given diaphragm surface area within a given mechanical amplitude, and thereby reducing the need for larger diaphragms moving through large mechanical amplitudes.

Otherwise, the loudspeaker requires a large surface area, often comprised of multiple radiators, moving through large mechanical amplitudes, to be able to displace enough volume of air to produce high sound pressure levels.

In other words, if you want it loud, you need a really big speaker.

Ceteris paribus, frequency modulation distortion and Doppler distortion, the forms most objectionable to a causal listener, vary directly with mechanical amplitude. Reducing mechanical amplitude reduces distortion.

Bottom Line: that Bluetooth speaker with its little speakers will sound awful.