The general rule is that the sound of the Vedas ought not to be changed, that there should be no tonal alterations. But there are rules permitting slight modifications based on the differences between the recensions - and these rules are according to the Siksa sastra. Slight tonal changes are also allowed. In some hymns of the Rgveda the "a-kara" and "e-kara" are drawn out further than in the other Vedas. In some recensions we have "m" and in some others "gm" - these are called "anusvara". The differences are not so much related to letters or syllables as they are tone and accent.
Sound means so much to the Vedic tradition, so due importance must be given to it. Thus Siksa sastra is the Vedapurusa's organ of breathing.
The 50 letters of the Sanskrit alphabet are derived from the Vedic sounds. If you add "jna" to them you will have 51. These letters are called "matrka". The word has more than one meaning. Importantly, "matr" or "mata" means Amba, the World Mother. The 51 letters make up her form - Amba, Parasakti, personifies them. If the cosmos is the creation of this Supreme Goddess and, if it is also remembered that creation was accomplished with sound, Amba must be the incarnation of the 51 letters. The Sakta Tantras declare that the 51 letters are the limbs of Amba and correlate the letters with different parts of her sacred body. The 51 Sakti pithas [seats of the Supreme Goddess] are associated with one or another of these letters.
If siksa is particularly esteemed as the breathing organ of the Vedapurusa, we must also remember that it is made more glorious by the fact that it sheds light on the 51 letters which personify Amba.
For a general background, please see here