Most research into language-music relationships has privileged language in the comparisons that it makes between the two domains. Music has generally been explored as though it were a sonic domain made up of complex patterns that can elicit aesthetic or hedonic responses, while studies of language are founded on its capacity to express complex propositions that can reflect states of affairs in the world. While music may resemble language in its combinatorial properties, in comparison with language it lacks the all-important property of compositionality; it thus appears to be a pale analogue of language with limited utility and little relevance outside the realm of entertainment. This view is, however, completely controverted by the fact that across cultures music is encountered as a participatory medium for communicative interaction with diverse and significant functions. Participatory music displays features and involves processes that equip it to manage social relations by inducing a sense of mutual affiliation between participants. At least some of those features and processes are present in other modes of human interaction, particularly those genres of speech concerned with establishing or continuing mutual affiliation or attachment, generally termed "phatic". I suggest that music as an interactive medium intersects so significantly with speech in the phatic register as to be indistinguishable from it. I hypothesise that affiliative communicative interaction need be neither music nor speech, but that these are best construed as culturally-constituted categories of human behaviour; the superordinate and generalisable category into which both fall is that of human affiliative communicative behaviour, which can be claimed in different cultures to be music, speech, or any one of a range of other categories in other possible taxonomies of human communicative behaviour. This paper will survey the evidence from ethnomusicology, linguistics and the cognitive sciences of music, and from recent collaborative research into spontaneous interaction in speech and music that lend support to this hypothesis.