Get some proper (Fischer) plasterboard fixing and screws.  The ones that come with the mounts are made of cheese.

The quality and condition of plasterboard varies greatly. Some contractors use absolutely wimpy materials. The anchors supplied with the speakers may not be appropriate for your plasterboard. Most hardware stores stock a variety of fasteners, each designed for a slightly different situation. I recommend using a type that expands and grips the wall from behind. Also, a given fastener is designed for a range of wall thickness.

If you have a “stud finder”, try to find a stud in a appropriate location.

The Era 300 wall mount description page specifically states that they must be drilled into studs.  They are NOT to be used with wallboard type anchors.  

https://www.sonos.com/en-us/shop/sonos-era-300-wall-mount

Stud mounting only (not for drywall)

Thanks all, I’ll give it a go with the recommended Fischer wall plugs and update here.

That’s not the recommendation of the manufacturer at all.  The official statement is the mounts are to be drilled into studs, no wallboard fasteners allowed.  You are risking damaging your speakers, not to mention creating big holes in your wall where the heavy speakers are going to pull out the wallboard fasteners like a cork out of a bottle.  

Oh! I was thinking about generic speaker mounts and did not check the details of this mount. As this is a single point mount, I don’t recommend drywall mounting. High quality 5/8” drywall in excellent condition might work out, but I wouldn’t risk it.

Another vote for ‘find the studs’.  And you don’t need a gadget with flashing LEDs etc.  I have had these and get very mixed results.  Get a small magnet.  When you find the screws/nails used to attach the plasterboard/drywall to the wooden studs the magnet will ‘stick’ to the wall.  Studs are generally placed a standard distance apart (may depend on where you are in the world and common practice).  Here in the UK they are 16 inches/40cm apart.  So once you find the first one you get a pretty accurate guess on where the adjacent one is.  Best not drill into the screw heads but above or below where you found the screws.