WALTERBORO, S.C. — Two childhood friends of Paul Murdaugh’s each took the witness stand Wednesday to tell a jury that a third voice heard on a cellphone video, recorded minutes before Paul and his mother, Maggie, were murdered, is the voice of his father, Alex Murdaugh. The 58-second video, played to the jury on the eighth day of Murdaugh’s double-murder trial, is vital prosecution evidence because, up to now, Murdaugh has contended that he was nowhere near the dog kennels the night his wife and youngest son were murdered. Placing Murdaugh at the kennels could puncture Murdaugh’s major alibi —…