One way of streaming music from a cell phone to loudspeakers is by using a Bluetooth amplifier. These units have Bluetooth reception as well as amplification built into a single unit which is usually quite small. However, one of the drawbacks of using such an integrated device is that the wattage is usually quite limited. This limitation stems from the fact that most of these Bluetooth receivers work with a fairly low supply voltage such as 12 V. As a result, when connecting speakers with an impedance of more than 4 ohms, the wattage is usually quite low because the voltage swing can be generated by the receiver is not sufficient to drive speakers loud enough.
Let me show you some ways for improving loudness in those cases. Obviously, the simplest solution is to purchase speakers that have a low impedance. Alternatively, you can also connect to speakers in parallel. However, this second solution is not ideal. That is because connecting two speakers in parallel can cause instability of the amplifier. This is because the impedance of each speaker varies over frequency and therefore the impedance that is seen as a total is not stable. They are obviously subtle differences between different speakers because of how the speakers are manufactured.
Another way to improve performance is to use a transformer. The transformer is going to lower the impedance of the speakers. In other words, the voltage that is being produced by the Bluetooth amplifier is being amplified. Such a transformer connects to the output of the amplifier and then to the speakers. Some higher and amplifiers actually have multiple outputs which are customized for different speaker impedances. Such amplifiers have transformers built into the amplifier.
However, be careful not to connect speakers with an impedance that is lower than the minimum impedance supported by the amplifier. Otherwise you can destroy your Bluetooth receiver. Another way to improve loudness is to connect speakers which are more sensitive. The higher the sensitivity of loudspeakers the less wattage you need to feed it to the speakers.