"soundless voice" is the question song to Hitoshizuku-P's hit song "proof of life", first uploaded on October 19th 2008. soundless voice was quite popular with over half a million views, and a month after it was uploaded, Hitoshizuku-P uploaded an "answer song" to tell Rin's version to the story. This song is Hitoshizuku-P's "P name origin"; in the lyrics of the song, a part of it said "Hitoshizuku"
It tells a story of Len, who watches over Rin. She was dying and Len was worried about her health condition. Unfortunately Rin's condition worsened and nothing can be done about it. When the time finally came, it was revealed later that Len have always wanted to tell her he loved her; though it was too late. In a matter of timeline, Len's version goes faster for Len is shown mourning Rin, whilst Rin's version stopped when she died.
In Hitoshizuku-P's album STORIA, a remastered track of proof of life, and also the third song to the mini series "endless wedge" is available, explaining more to what happened after Rin died. "soundless voice" also became one of the Append demos for Len (power, cold).