DIAGNOSIS Network latency. The speakers are on different Wi-Fi bands (2.4GHz vs. 5GHz) or are too far from the router, causing them to receive and process the audio stream out of sync.
FIX Standardize their connections. Ensure all speakers in the group are connected to the same Wi-Fi band—preferably 5GHz for lower latency, if they all support it and have a strong signal. Recreate the group after making this change.
IF THAT DIDN'T WORK The problem is often one outlier device. In the Alexa app, delete the existing multi-room group. Create a new group, adding devices one by one and testing sync after each addition. You'll identify which specific Echo (often an older model) is causing the lag.
HEADS UP Multi-room music will never be sample-accurate sync; a slight millisecond delay is normal. But a noticeable echo or words doubling is a network issue. Wired Echos (like an Echo Link) will always perform better as the group source.