Some estimates which I found worth sharing:

1. For fun, I searched for Chixculub TNT equivalent and e.g. ScienceDaily claims

   The energy released by the impact that blew out the Chicxulub crater was equivalent to about 100 million megatons, many orders of magnitude greater than the nuclear explosion at Hiroshima, a 15-kiloton blast.

   This seems to match up what WolframAlpha uses to rank TNT equivalents.

2. There is a Noise Prediction Calculator which estimates the safe distance from a blast:

   This calculator can be used to predict the distance from an open detonation at which 140 decibels (dB) could be expected to be achieved. The 140dB level is widely used as a "safety cutoff" for exposure to impulsive noises while using hearing protection.

   Plugging in $10^{17} {\rm kg}$ (of TNT) in that formula (which is probably a rather crude estimate) gives ${460\,320 \rm km} \approx 1.4 \cdot (2 \pi a_\oplus)$ (with $a_\oplus$ being the Earth's radius), meaning more than equatorial circumference of our planet. In other words, no where on Earth was it safe according to UN safety standards.

So let us approach it the other way round: What is the loudest possible sound?

Strictly speaking, the loudest possible sound in air, is 194 dB. The “loudness” of the sound is dictated by how large the amplitude of the waves is compared to ambient air pressure. A sound of 194 dB has a pressure deviation of 101.325 kPa, which is ambient pressure at sea level, at 0 degrees Celsius (32 Fahrenheit). Essentially, at 194 dB, the waves are creating a complete vacuum between themselves.

You can go louder than 194 dB, but that’s not technically a “sound” anymore. The extra energy starts distorting the entire wave, and you end up with something that’s more a shockwave and less a soundwave. At that level, sounds don’t pass through air — they push the air along, producing pressurized burst (shockwaves).

On the shockwave of Chicxulub, you can e.g. an page at the Lunar and Planetary Institute.

The Chicxulub Impact event produced a shock wave and air blast that radiated across the seas, over coastlines, and deep into the continental interior. Winds far in excess of 1000 kilometers per hour were possible near the impact site, although they decreased with distance from the impact site. The pressure pulse and winds would have scoured soils and shredded vegetation and any animals living in nearby ecosystems. An initial estimate of the area damaged by an air blast was a radius 1500 kilometers. There are several factors that can affect this estimate, so the uncertainty might be better reflected in a range of radii from ~900 to ~1800 km. The travel times are quite short, so this effect would have occurred in advance of any falling debris ejected from the Chicxulub crater.

Some estimates which I found worth sharing:

1. For fun, I searched for Chixculub TNT equivalent and e.g. ScienceDaily claims

   The energy released by the impact that blew out the Chicxulub crater was equivalent to about 100 million megatons, many orders of magnitude greater than the nuclear explosion at Hiroshima, a 15-kiloton blast.

   This seems to match up what WolframAlpha uses to rank TNT equivalents.

2. There is a Noise Prediction Calculator which estimates the safe distance from a blast:

   This calculator can be used to predict the distance from an open detonation at which 140 decibels (dB) could be expected to be achieved. The 140dB level is widely used as a "safety cutoff" for exposure to impulsive noises while using hearing protection.

   Plugging in $10^{17} {\rm kg}$ (of TNT) in that formula (which is probably a rather crude estimate) gives ${460\,320 \rm km} \approx 1.4 \cdot (2 \pi a_\oplus)$ (with $a_\oplus$ being the Earth's radius), meaning more than equatorial circumference of our planet. In other words, no where on Earth was it safe according to UN safety standards.

So let us approach it the other way round: What is the loudest possible sound?

Strictly speaking, the loudest possible sound in air, is 194 dB. The “loudness” of the sound is dictated by how large the amplitude of the waves is compared to ambient air pressure. A sound of 194 dB has a pressure deviation of 101.325 kPa, which is ambient pressure at sea level, at 0 degrees Celsius (32 Fahrenheit). Essentially, at 194 dB, the waves are creating a complete vacuum between themselves.

You can go louder than 194 dB, but that’s not technically a “sound” anymore. The extra energy starts distorting the entire wave, and you end up with something that’s more a shockwave and less a soundwave. At that level, sounds don’t pass through air — they push the air along, producing pressurized burst (shockwaves).

On the shockwave of Chicxulub, I found the page at the Lunar and Planetary Institute insightful:

The Chicxulub Impact event produced a shock wave and air blast that radiated across the seas, over coastlines, and deep into the continental interior. Winds far in excess of 1000 kilometers per hour were possible near the impact site, although they decreased with distance from the impact site. The pressure pulse and winds would have scoured soils and shredded vegetation and any animals living in nearby ecosystems. An initial estimate of the area damaged by an air blast was a radius 1500 kilometers. There are several factors that can affect this estimate, so the uncertainty might be better reflected in a range of radii from ~900 to ~1800 km. The travel times are quite short, so this effect would have occurred in advance of any falling debris ejected from the Chicxulub crater.

Equipped with all this, we can now try to answer "How loud was the Chixculub explosion?" even if you did not specify where you measure loudness: At the radius of $\approx 1\,500 {\rm km}$ from the epicenter, the shockwaves "fades" out to become the loudest possible sound of 194dB.