alsa-sys links libasound at BUILD time (every other windowing/audio crate here dlopens at run time). Its build script finds the library with pkg-config and says so in cargo:rustc-link-lib / rustc-link-search, which buck2's buildscript runner drops unless asked to keep them - leaving snd_pcm_* undefined at the final link.

6[buildscript.run]
7rustc_link_lib = true
8rustc_link_search = true