The eruption is listed as occurring at 4:15:45 (4.26) UTC. KLAX is 8550 km from Tonga. The pressure peak at KLAX is 11.9 UTC. The computed speed of the pressure wave is 310 m/s, in good agreement with your value of 300 m/s. The speed of sound is 310 m/s at about 7000 m altitude.
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