most of the internal APIs in JDK 9 are inaccessible to code outside the JDK. Please seeJEP 260: Encapsulate Most Internal APIs for details and potential workarounds.
Principal Product Manager
Java Platform Group @ Oracle
But it seems ([Audio-engine-dev] Gervill 0.2 - Grokbase ) like it was actually planned to place the AudioSynthesizer in public (i.e. javax.sound.midi) package. Can you elaborate on it? Is there any reasony why Audio- or SoftSynthesizer shall NOT be available publicly?