Interesting. I don't hear anything when I use the -a flag.

Let's check with "reveal codes":

    $ ping -a 127.0.0.1 | less

    PING 127.0.0.1 (127.0.0.1) 56(84) bytes of data.

    64 bytes from 127.0.0.1: icmp_seq=1 ttl=64 time=0.029 ms^G

    64 bytes from 127.0.0.1: icmp_seq=2 ttl=64 time=0.023 ms^G

Looks like you need a terminal that has a BEL sound.

It just echoes a Bell character to your terminal. Make sure your terminal emulator supports it, and is configured properly: https://en.wikipedia.org/wiki/Bell_character

Windows Terminal shows a "visual bell" in the tab bar whenever BEL is emitted (even though I have it set to "audible" in advanced settings of the profile, hmm).

Edit: Uhm, right. I have also silenced all system sounds in Windows.

Whoa, nifty.

Why does firefox shows the "chose an application to open this link" when you access the http://localhost link, but shows the content correctly when you access the 127.0.0.1/0.0.0.0 ip-based version?

Browser expects to see some HTTP headers before the content. Adding HTTP version with response code, content type declaration and empty line dividing headers and content should do the trick:

    HTTP/1.1 200
    Content-Type: text/plan;

    1st line of content

TIL. thanx for -a.