Check your instrument cable and make sure it isn't compromised. Also, if you are using a standard instrument cable directly into the Alesis, make sure the Alesis deals with a TS plug correctly. If you have a DI box or amp head in between, either of these could be the issue as well. And finally but perhaps firstly, the pickups on the bass itself might be the issue. Hiss is unlikely to be the Alesis despite my dislike of the Multimix line and it certainly isn't Sonar.

well i've checked all cables, and even tried recording guitar not bass. and i still got the hiss :/ and now despite my best efforts with this mixer, my computer decides to not recognize it anymore. gaaah i cant wait for my tax return so i can get a real interface. anyone know what to do when the comp wont recognize the mixer? there are no drivers for this on the alesis website

Close the audio program. Unplug the usb cable. Turn off the mixer. Count to ten in language of choice. Turn on mixer. Plug in usb cable. Start audio program.

In the FWIW category, the whole Multimix line is known to be buggy.