Two audio amplifiers in series (IKEA symfonisk into powered speakers)
There are a few potential problems:


The output of the Sonos will be several volts. The input to your powered speaker will be &lt; 1 V max and exceeding this will result in clipping distortion.
Turning down the volume of the Sonos will prevent the clipping but since the volume control will precede the power amplifier the hum and noise from the Sonos amplifier stage will not be attenuated. i.e., the signal (music) to noise ratio will decrease.




Figure 1. A bridge-mode amplifier configuration. Note that neither side of the speaker is grounded. (Ignore the 160 W reference.) Image source lost in the mists of time.


The Sonos might be using a bridge-mode output which means that neither end of its speaker is grounded. That means that you have a problem with the COM / GND connection between the two systems.  For more see my answer to Single-supply amplifier design without output capacitor?.


Figure out the details of the Sonos, have a look at the power amplifier chips, read the datasheets and post the details in your question.