Pxls sounds
Back on April 14th 2017, an old Pxls staff member named Ludus Light sent a message in the Pxls Discord asking if there should be a sound that plays when you have a pixel ready. A user named foxc*m replied with a link to a Google Drive with some sound effects they designed a long time before they sent their message and suggested they could be used, which ended up being the Notify and Place sound effects that are still in use on the site to this day.[1]
On August 1st 2019 when the Pxls chat was added, a new sound effect was added alongside it that was designed by Atomic10.
Official
Chat Notify.wav
Plays when you receive a ping in the Pxls chat.
Notify.wav
Plays when you gain a pixel. Originally named "ui-select-ok.wav" before being added to Pxls.
Place.wav
Plays when you place a pixel. Originally named "ui-change-selection-ok.wav" before being added to Pxls.
Unused
In the Google Drive that foxc*m linked, there were several other sound effects that were never used on Pxls.
ui-notify.wav
ui-select-back.wav
ui-select-bad.wav
ui-select-start.wav
Custom notification sounds
magic.mp3 - Unknown
The My Little Pony: Friendship is Magic theme.
megalonotif.wav - Unknown
Megalovania from Undertale.
rick.wav - Unknown
Rick Astley - Never Gonna Give You Up.
thenotif.wav - Unknown
The Lick.
Vine Boom - Ayden
A custom sound of the Vine Boom meme.
Remixes
Some custom songs have been made using some of the Pxls sound effects.
Pixels Ready - Distance
Source: https://www.newgrounds.com/audio/listen/1071402
sneaky.mp3 - Atomic10
Based on the Chat Notify sound. Shared as an mp3 file in the Pxls Discord.