can you send its Value. also attachments should are Variant so you must use Variant on your function/sub declaration:
Call SendCDOMail(Me.txt_To, Me.txt_Subject, Me.txt_Msg, Me.txt_Cc, Me.txt_Attach)
Public Function SendCDOMail(txt_To As String, txt_Subject As String, txt_Msg As String, txt_Cc As String, txt_Attach As Variant)