When it comes to engaging your voice in magical work, it can be difficult to give a definitive answer as it depends a lot on the individual. The voice an be considered as a tool of manifestation. It takes the immaterial nature of thought and through intent transforms it into reality as sound.
Because every individual expresses themselves differently, each person will have their own voice to use. And that voice may be further colored by the individual practice or tradition. Some practices include ideas on the energy and influence of rhyme. Or tone. Some call for using song or chant to build a resonance of energy. Others call for finding your own individual tone that carries strength or authority.
In the end, what seems to matter the most is finding what you feel the strongest connection with. As long as what you do connects you to your magic then use it. Personally I like to keep things simple. So I use English, and use a declarative tone that speaks from both the head and the heart. Tone-wise I focus on speaking with an open and clear throat, and an engaged diaphram. My voice tends to turn slightly deeper, but there isn't any significant change.
Unfortunately I am not sure of any single link to go to. I came across my own information in bits and pieces that I picked up while reading about other topics.
I might suggest doing some searches for keywords like 'spellsong', 'Energy and voice', 'Mudras', and 'how to use magical chant'. It should at least pull up a few tidbits that will help you get started.
A lot of the specifics can vary as well, depending on what practice you are researching through. I think researching Mudras, Tibetan chant, and Buddhist mantra can help because they have pretty good overlap. Numerology can also help because different tones and sounds are believed to coincide with different numbers as well. ... It is all about frequency and vibration after all.
