In some Mantras these sounds though invisible are embedded in Mantras and stotras by Great Acharyas.

Soundaryalahari By Adi Shankaracharya contains the 51 Bheejaksharas, the Seed sounds of Devi , they are embedded in the text though it is invisible.

Each God is assigned specific numbers of letters, called Aksharas,

Interesting to note that Akshara, loosely translated as Letters means Limitless, therby signifying the power of sound produced by each letter.