Think this in terms of sound level:

https://www.youtube.com/shorts/IUENFTc79pA

These aircraft might have been unpressurised, but they would have been flown low. 12000 ft would have been the maximum, but in most cases they would have been flown two, three, maybe four-thousand feet above ground level.

These aircraft also had cabin heaters (usually petrol-fired ones) that would have provided some kind of heat.

Pressurised aircraft would actually come in quite soon after that.

The first pressurised airliner was the Belgian Renard R.35 prototype, which had been ordered by the Belgian airline SABENA in 1936 and was completed in early 1938; however the sole prototype was destroyed in an unauthorised test flight (that was only supposed to be a fast ground run, the aircraft hadn't been cleared for flight yet) on April 1st 1938. Already having submitted firm orders for the DC-3, SABENA cancelled its order for the Renard R.35 and the company abandoned its development.

The first pressurised airliner to actually enter service was the Boeing 307 Stratoliner, which first flew in December of 1938 and entered service in July of 1940. It was Boeing's reply to Douglas DC-4E, an airliner which had been in development since 1935 and which, while the prototype was unpressurised, had been envisaged with a pressurised cabin in its production form. While the DC-4E was a failure and never flew with a pressure cabin, the Boeing 307 did, although only 10 were built as Boeing was forced to focus on the production of the B-17 with the onset of war (the Stratoliner shared the wings, engine nacelles and tailplanes of the early B-17s).