Old Noldorin has a couple of clear examples in which [tʰm] > [tʰw]:

• ᴹ✶pathmā > ON. pathwa > N. pathw “level place, sward” (Ety/PATH).
• ᴹ✶khithme [> *khithwe] > N. hithw “level place, sward” (Ety/KHIS).

There is also a indication that [kʰm] > [kʰw] in notes associated with the Old Noldorin use of the Feanorian Alphabet from the 1930s:

Signs of the change 9 [ɧ] > χ^ı̯ medially are seen in the confusion in late ON of medial dÔ = khy and 9, and in the occasional appearance of initial dÔ after the article `B [i] “the” for 9. Similarly a sign of change of o  [ƕ] > χ^u̯ is seen in the use of this sign instead of dé or dn for the rare khw (derived from PQ khm) [emphasis added] (PE22/29).

Whether this sound change also applied to [pʰm] is unclear, as is whether or not this change could occur in later Sindarin also.

See OS. [m] became [w] after aspirates for discussion.