Really confusing float question

The physics has detected a collision with the ground, and because the ground is solid, it unembeds the capsule from it. Then on the next frame, the capsule falls under gravity again and collides with the ground. This is how the code makes you appear to "land" on solid surfaces.

You need to store some internal state so that you can tell the difference between standing on the floor, and landing on the floor after a fall. Luckily you already have it! Try this:

     void OnCollisionEnter(Collision collision)
     {
         if (!isgrounded && collision.gameObject.tag == "Ground")
         {
             audiosource.PlayOneShot(hitground);
             Debug.Log("xd");
         }
     }