9. A computer implemented method comprising: under control of one or more computing devices, generating audio content based at least in part on textual content; causing output of the generated audio content; causing presentation of the textual content; maintaining synchronization between an output pointer of textual content being presented and an associated position of the generated audio content being output, wherein the associated position of the generated audio content advances during output of the generated audio content, and wherein the output pointer indicates a position within the textual content corresponding to a current output position of the generated audio content; obtaining an input pointer referencing a location within the textual content being presented from an input device; during advancement of the output pointer, determining, independent of the obtained input pointer, a position of the output pointer corresponding to the current output position of the generated audio content; determining a segment of textual content based at least in part on a difference between the determined position of the output pointer and the location within the textual content referenced by the input pointer; determining a length of time required to output audio content corresponding to the determined segment of textual content; and modifying an attribute associated with the output of the generated audio content based at least in part on the determined length of time.
9. A computer implemented method comprising: under control of one or more computing devices, generating audio content based at least in part on textual content; causing output of the generated audio content; causing presentation of the textual content; maintaining synchronization between an output pointer of textual content being presented and an associated position of the generated audio content being output, wherein the associated position of the generated audio content advances during output of the generated audio content, and wherein the output pointer indicates a position within the textual content corresponding to a current output position of the generated audio content; obtaining an input pointer referencing a location within the textual content being presented from an input device; during advancement of the output pointer, determining, independent of the obtained input pointer, a position of the output pointer corresponding to the current output position of the generated audio content; determining a segment of textual content based at least in part on a difference between the determined position of the output pointer and the location within the textual content referenced by the input pointer; determining a length of time required to output audio content corresponding to the determined segment of textual content; and modifying an attribute associated with the output of the generated audio content based at least in part on the determined length of time. 11. The computer-implemented method of claim 9 , wherein obtaining the input pointer referencing the location within the textual content comprises obtaining a location corresponding to at least one of a natural feature or a predefined feature of the textual content.