When linking project by add_subdirectory(libsoundio_dir) project's include directories are not being propagated to linking project thus giving compilation errors. This can be fixed if in CMakeLists.txt:

include_directories(
   ${libsoundio_SOURCE_DIR}
   ${libsoundio_BINARY_DIR}
   "${libsoundio_SOURCE_DIR}/test"
   "${libsoundio_SOURCE_DIR}/src"

include_directores will be changed to:

target_include_directories(
   soundio
   ${libsoundio_SOURCE_DIR}
   ${libsoundio_BINARY_DIR}
   "${libsoundio_SOURCE_DIR}/test"
   "${libsoundio_SOURCE_DIR}/src"

and moved after add_library line. In order not to open too much interface to linking projects following line can be added after add_library instead of changing:

target_include_directories(
   soundio
   ${libsoundio_SOURCE_DIR}
)

Adding include directory to root of project and moving public includes there adds more protection to includes:

target_include_directories(
   soundio
   ${libsoundio_SOURCE_DIR}/include
)