Tanchuma Pekudei 9: When we do not have Mishkan or Mikdash, the prayer of out mouths should be like voluntary offerings.

Rashi: A matter of appeasement that my mouth offers to You. Nedavah is always an expression of acceptance.

Radak: The Tefilos and praises that I say in front of You with a spirit of volunteering, should be acceptable to You.

Malbim: What I volunteered with my mouth via a Shevu'ah to guard Mishpat.

Radak: Yes - most of my Tefilah is for this.

Malbim: Yes - in the merit of the Shevu'ah that I volunteered with my mouth, teach to me Your Mishpatim, so I will know to judge Your nation, to understand between good and evil.