tl;dr: The exact answer is currently unknown (to me), possibly it is untestable (as aircraft have a flight ceiling so would not normally go above a certain height anyway). Possibly it can be modelled (but not by me!).
41,000 feet = 7.7 miles
18,000 feet = 3.4 miles
A large jet aircraft flying overhead at 41,000 feet is perfectly audible from the ground - but it helps if it's otherwise reasonably quiet outside. Few large jets seem to fly much above 39,000 feet so I'm not sure if this is a common occurrence. I only ever check to see what's flying above me when I hear it so it's technically possible that the sound from a higher-flying large jet wouldn't be able to reach me (and so I wouldn't know that they're there) but I don't think they fly much higher than 41,000 feet as that seems to be their comfortable limit.
FlightRadar24 and ADS-B Exchange both colour code information about the height of a plane into their live maps. With FR24 the aircraft icons are the same colour but clicking on any shows its previous route and the colour changes (e.g. white is on the ground then it goes yellow as it starts to climb). ADS-B colours the aircraft icon so you can see at a glance if planes are coloured a vibrant pinky purple (those are going over at 45,000 feet). The ones going over at that height are generally the business jet types, much smaller and quieter than the bigger jetliners.
FlightRadar24 tracking an ATR-72-600 arriving at Heathrow. Zooming out shows the change in colour (darker blue = higher, paler green = lower) throughout its flight from the Isle of Man. All aircraft icons are the same yellow colour.
ADS-B Exchange showing the same flight. The colour of the plane's icon indicates that it's travelling at around 3,000 feet and the green in the path trail behind it shows that it was previously higher. The pinkish aircraft near top left heading North was travelling at around 36,000 feet.
If a massive jetliner were able to fly over at 45,000 would I be able to hear it? Or 60,000 feet? (Assuming optimal conditions such as late at night when other ground noise is quieter and fewer other aircraft, air temperature etc). At the moment I don't know but of course it's moot as they just don't seem to fly that high. So 41,000 is the practical limit for aircraft I'm likely to hear.
Some military jets do fly much higher but they don't seem to be doing much of it over London where I am so I can't assess how audible they are.
Not that I've been keeping records or anything... but two aircraft that I've heard at 41,000 were Boeing 747-8R7F Cargolux (on 8 Feb 2024) and an Airbus A330-243 (21 May 2024). A week apart in May 2022 I heard two Boeing 787-9 Dreamliner going over at 39,000 feet and in June that year heard a Boeing 747-4R7F going over at 35,000.
My favourite aircraft to hear have propellers. These usually fly over between 18,000 and 25,000 feet and are very audible for a longer period of time (they're flying more slowly than the massive jets so take longer to pass). I've heard several of my beloved Hercules* C-130s going over up to 25,000 feet and the Airbus A400M Atlas going over at 19,000 feet. Another favourite is the Antonov fleet and I get the An-26B and An-12B going over at 19,000 to 21,000 feet - they are super loud (famously so, go and have a listen on YouTube).
*The Royal Air Force has swapped from Hercules to Airbus A400M Atlas though I don't have the audio acuity to be able to tell which is which by sound (possibly I could if I heard them in quick succession). So no more RAF Hercs going over but USA, Canada, Saudi and other militaries are still using them and so they do regularly appear in our airspace. Hooray!
