The best way to reduce the delay is to use the digital input on the Beam. There is no way to reduce the delay on a Sonos analog input below 75ms.

The only input option is HDMI. I’m going into that. Is there another input I’m not aware of?

The HDMI ARC input? That’s the lowest latency available, but it shouldn’t be 75ms, unless you’re then grouping the Beam with other Sonos speakers.  

Ah I see. I don’t have it grouped so there shouldn’t be a delay but there is. It’s very minimal but ideally there would be none. Maybe it’s sonos’s internal processing causing the delay? 

The delay is, I think, about 35ms. The Beam doesn't do much audio processing.  It is possible that the lag, or part of it, is there to allow sync will surround speakers when added.

I'm afraid that the Sonos system is designed as a multiroom hifi and AV system, not as a speaker for a musical instrument. 

Any internal processing should be minimal, sub 5 ms as a guess, and likely not detectable, since it’s designed for syncing with video screens. I’d be looking at whatever device you have feeding the HDMI-ARC signal to the Beam.