Inspired by this answer by Arnaud Fournet I have this question: Do Old Indic (Vedic, Sankrit) words beginning with a voiceless aspirated stop (like ph, th, or kh) have cognates in other branches of Indogermanic? What are typical regular sound correspondences?
As a rule, the voiceless aspirates are a dialectal feature. In most languages, they cannot be distinguished from plain voiceless. As regards *kh, the typical pattern is Old Indian kh, Armenian x, Greek k(h). For example, in a potentially onomatopeic verb: *kakh- "to laugh", where probably no laryngeal is involved. As regards *th, typically Old Indian th, Armenian t, Greek t(h), very often preceded by initial s- as in *st(h)eH- "to stand", cf. *pont(H)- "path, way through", *ost(H)- "bone". As regards *ph, typically Old Indian ph, Armenian ph, Greek p(h), a good example is *(s-)phol- "to fall".