You will not be able to keep the Yamaha and the Sonos Amp connected at the same time to the same set of speakers to add power to push those speakers. The Sonos Amp is 125 watts per channel at 8 ohms which would likely be a power improvement over your current receiver but would lack the ability to output to the other 7 channels the Yamaha supports.

The Play:1 will group with everything on a Sonos system and play music in sync throughout the home. However, the Play:1 can not be stereo paired with a Sonos One or used as rears in a home theater configuration with a Sonos One as the opposite rear speaker.

If you are planning on using passive, unpowered speakers you would need to use the Sonos Amp. The Connect does not have an amplifier and is unable to drive passive speakers. The great news is that the Sonos Amp includes an HDMI port and can be used as the sound solution for a television while still playing all of your streaming sound content when the television is off!

In general if the audio is cutting out from the television source there is something that the television is doing that changes the sound output. This could be manually switching sources, the television auto detecting an HDMI device and attempting to change output, among other issues. Make sure that your television is set to output sound through the digital audio port as its default sound selection. There may be devices (set top boxes, video game consoles, etc) that are connected through HDMI that change that setting when powered on through CEC+ controls which may change the sound output. You can rule this out by turning off CEC+ in your advanced menu but that may have other implications with how your system works. If you have not tried to contact Sonos Customer Experience on Twitter @sonos give that a try or try a different way to contact them from the options at sonos.com/support.