He was wounded on or before Aug. 31, 1864, as the company muster roll made out that day (or apparently made out on that date) lists him as "absent, wounded" without specifying when and where he was wounded.
Mary signed a power of attorney (in order to receive money due her deceased husband) on or before November 29, 1864, so William was clearly dead before that date.
I regret that, looking at all the records I could find for him, I could not find sufficient information to ascertain when and where he was wounded, when and where he died, or when and where he was buried.
I wish I could have helped you more.
Do note, however, that William is listed as "William," with no middle name, while there is a "Malcolm" in the same regiment who died in 1862 in hospital at Lauderdale Springs, MS, in 1862 and is likely buried at the large and beautiful Lauderdale Springs CS Cemetery. Of course, I do not know if these two men are related. -- Jim Huffman, Gainesville Vols, SCV Camp 373, Pearl River County, MS