This is the classic study demonstrating (at least one) suboscine to have innate vocalisations. With Empidonax, a tyrant flycatcher (Tyrannidae) You steal an egg, raise the chick, and play the song of a related species at it repeatedly. Outcome: it sings like its parents did, despite never having heard them:
https://academic.oup.com/auk/article-abstract/101/1/13/5191285?redirectedFrom=fulltext
The same holds true for some antbirds (Thamnophilidae):
https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0095746
The same does not work so well for Bellbirds, another suboscine (Cotingidae). I suspect this may be an exceptional instance among suboscines, given how unusual bellbird calls are for the group, but probably there are others like this remaining to be found.
https://www.researchgate.net/public...e_Suboscine_Bellbirds_Procnias_spp_Cotingidae
For oscines (and any other cases of suboscines that learn), the thinking is that voices can be learned. However, there is strong geographical variation in voice. And some traits - e.g. the ability to trill very quickly, the highest and lowest notes that a bird can attain, the quality of timbre of sound - MUST be defined physiologically to some extent. I don't think you could teach a warbler to make a sound like an owl.
https://www.researchgate.net/public...hree-striped_Warbler_Basileuterus_tristriatus
I don't know if things have moved on since 2014, but this summarises where they were then in the context of this particular study, for oscines.
"Voice has been widely used to delimit species in suboscine passerines (in which vocalisations are considered innate: Kroodsma 1984, Isler et al. 1998), but is relatively little used for Neotropical oscines. Vocal differences are nonetheless evident between allopatric oscine populations (e.g.
Cadena et al. 2007, Donegan & Avendaño 2010). Gape muscle strength and bill mass must influence the vocal repertoire of oscines, e.g. their ability to trill at given speeds or deliver notes of certain lengths or frequencies (e.g. Podos et al. 2004), and vocal differentiation is considered relevant to species limits in all birds (e.g. Helbig et al. 2002, Tobias et al. 2010).
It is unclear whether all vocal variables found here to be differentiated reflect
physiological or innate differences between populations. For example, from this study it appears that most B. tristriatus populations avoid acoustic frequencies used by crickets, cicadas or other insects, by singing at a higher pitch. It is perhaps no coincidence that the ‘highest frequency’ population (in San Lucas) has some of the highest frequency insects (max. 4.9–6.1 kHz vs. 4.2–4.6 kHz for Central Andes populations in a sample of slow songs)
or that the lowest frequency songs (in Bolivia) rarely have insect calls vocalising together (and when insect calls are present, the rising song ‘crosses the insect line’: ML132723). It is unclear whether differences in usage of low frequencies are learned / behavioural or innate.
Other differences, in repertoire, song structure, song speed and max. frequencies (which are unconstrained by insects) are probably reflective of physiological or innate differences."
As a "former amateur ornithologist", I've not been following this one recently though, maybe there are some novel developments since.