You need to look at the acceptable voltage range for your speaker set. They
usually specify a "±".
12V may be acceptable.

Typically I've seen 13+ VDC called out because of the use of a linear
regulator. Typically at least a 0.7V margine is required to regulate to
12VDC (so input of greater than 12.7VDC).

....but again, check out the input DC requirement.

Also note the power limitation on your PS:

ORIGINAL HARDWA
Input 120 VAC 60Hz 50W
Output 13.5VDC 2.5A

- 50W. .. actually more like 35W


POSSIBLE REPLACEMENT
Input 120V AC 60Hz 23W
Output 12VDC 1000mA

- 23W ... more like 15 W to your system.