This "randomly" works because of how DCS loads assets. If DCS loads "Saved Games/DCS.openbeta/Sounds" after the original sound, then it works, because the original is overwritten in the RAM.
If you check your dcs.log after a DCS startup in which the /Sounds worked, you will see that your custom sounds was loaded after the originals.
2025-10-23 10:56:40.027 INFO SOUND (7156): Added sound path: ./Mods/aircraft/Bf-109K-4/Sounds
2025-10-23 10:56:40.544 INFO SOUND (Main): Added sound path: C:\Users\User\Saved Games\DCS.openbeta\Sounds
You can make this happen every time by making changes in the "Saved Games/DCS.openbeta/Mods" directory, like a changed time stamp or an added/removed file. This will make DCS linger for enough time load the original sound, since the /Mods are loaded before /Sounds. Then the original sounds are overwritten by /Sounds.
You can also make a .bat-file that helps you with this, making changes to the /Mods and starting DCS with on execution.
Unfortunately DCS must be restarted for it to work, so you have to start it twice per session to be sure to have your own /sounds.