If "accompaniment" refers to something that goes along with something else:
* Isolation: This implies being alone and not having any company.
* Separation: This suggests being apart from something else, rather than together.
* Independence: This means being self-sufficient and not relying on anything else.
If "accompaniment" refers to music played alongside something else:
* Solo: This refers to music played by a single performer, without any additional instruments.
* Unaccompanied: This means performing without any musical support.
If "accompaniment" refers to a person who accompanies someone else:
* Solo traveler: This refers to someone who travels alone.
* Unattended: This means being without someone accompanying you.
The best antonym will depend on the specific context and the meaning you want to convey.