Afterthought
Puritan Board Senior
Don't forget that the tongue and voice are part of worshipping God with your body. Standing is also an appropriate posture that should be counted as part of worshipping God with the body. Bowing the head and closing the eyes--our typical (though not only) cultural form for prayer even as lifting hands was back in the Old Testament--are also worshipping God with the body in external signs. These are externally expressive and should be viewed as such.
Even so, the key thing with singing is not the voice but that the heart is ordered properly and the singing is done with grace. You could sing well and offer offensive worship to God; you could sing poorly and offer pleasing worship to the Lord; if you seek to sing with grace it is likely the voice will follow to a decent level (especially if you sing acapella) for many, but the heart is the spiritual worship the Lord seeks. Likewise with prayer.
Even so, the key thing with singing is not the voice but that the heart is ordered properly and the singing is done with grace. You could sing well and offer offensive worship to God; you could sing poorly and offer pleasing worship to the Lord; if you seek to sing with grace it is likely the voice will follow to a decent level (especially if you sing acapella) for many, but the heart is the spiritual worship the Lord seeks. Likewise with prayer.