In order to audit for members of the sysadmin fixed server role, we will need to use two system views: sys.server_role_members and sys.server_principals.
SELECT sp.principal_id
FROM sys.server_role_members srm
INNER JOIN sys.server_principals sp
ON srm.member_principal_id = sp.principal_id
WHERE srm.role_principal_id = (
SELECT principal_id
FROM sys.server_principals
WHERE [Name] = 'sysadmin')