The published letters of performers, composers, critics, and others involved with music can be found by adding the keyword correspondence when using OneSearch. If a singer worked closely with a particular composer, you may find relevant items in the latter's correspondence. You may also find letters quoted, in part or in full, in composer biographies in the ML410 section of the library, and in singer biographies in ML420.
Another place to track down letters and other documents is ArchiveGrid. You may find some collections that have been digitized and made available online.
Concert reviews can be found by using indexes of newspapers and the musical press. In some cases you will be able to limit your results to reviews. In addition to modern journals, try these historical sources:
You can also find the reviews and music criticism of specific authors by searching the library catalog for the subject musical criticism.
The archives of theaters, concert halls, publishers, and other institutions may contain unique and informative documents related to the creation, performance, and dissemination of music. Here are a few that have made their materials available online:
It's not unusual to find recordings of modern opera singers, but Parsons Music Library also has a number of recordings made by singers of (fairly) long ago. Some are on CD, but others are only available as LPs (vinyl), so when you look them up in OneSearch, don't limit to music CDs.
YouTube is also an excellent place to look for historical recordings of opera singers, but be warned that the sound quality -- often poor to begin with -- is typically worse than the LP or CD that it was digitized from.