Helmi_S,
Sounds either like it could be the TV, or possibly the connecting cables. I would try powering off (no lights) the TV, ATV and Sonos HT device and re-seating the cables to begin with.
Some older TV’s might not be able to process the audio through their ports quickly enough, so ensure any sound enhancements that your TV ‘may' support, in its settings, are switched off.
If your TV is a newer model then it may support audio pass-through on both it’s HDMI-ARC and Optical ports… check your TV Settings and if that feature is supported, then I would enable that and ensure all its connected peripherals are sending a Dolby Digital audio signal. If not then perhaps try the following suggestion instead...
A good test with some older TV’s is to try an audio splitter/extractor between the ATV and the Sonos device and see if that improves things by cutting out the 'pass-through/processing’ of the audio through the TV altogether. Here’s are various examples of such devices depending on how many devices you may wish to connect to the TV, and type of TV etc.
https://www.amazon.com/NewBEP-Switcher-Selector-Extractor-Splitter/dp/B07438WJ8G/ref=sr_1_7
https://www.amazon.com/AV-Access-Extractor-Splitter-Amplifier/dp/B07D9BNVM1/ref=sr_1_21_sspa
https://www.amazon.com/Extractor-Koopman-Optical-Converter-Supports/dp/B082KW5MXS/ref=asc_df_B082KW5MXS
Another means of checking if the TV is struggling with the audio, is to downgrade the type audio itself passing through the TV from Dolby Surround Sound to Stereo PCM and see if that audio format passes to the Sonos HT device without any delay or stutter, If so, then the audio splitter/extractor may just be your best bet, after trying the various other things mentioned.
Hope that assists. 