I can't verify that this will work without at least on set of JSON data to work with, but give it a try. If it doesn't work, post the JSON you used to create the $audit object:
$details = $audit |
Select-Object UserId,
ObjectID,
CreationTime,
@{n='oldlabel';e={$oldlabel}},
@{n='newlabel';e={$newlabel}},
SensitivityLabelJustificationText,
@{n='OldSensitivityLabelOwnerEmail';e={$_.Sensitivitylabeleventdata.OldSensitivityLabelOwnerEmail}},
@{n='SensitivityLabelOwnerEmail';e={$_.Sensitivitylabeleventdata.SensitivityLabelOwnerEmail}}