The manual for this radio (in Adobe .pdf format) is available for download
directly from Icom America, free of charges.
Since the "AquaQuake" function works and you do hear static when the squelch is off, that is sufficient to let us know that the voice coil of the speaker is functional and should
produce sounds with the right signal applied.
The first thing to determine is if you're close enough to another operating transmitter for the radio to receive a signal. One of the 10 weather channels the radio is capable of tuning should be close enough produce some intelligible audio from the speaker - assuming the squelch setting isn't set too high.
If a WX channel works, then you'll need to see if there's a VHF Marine band transmitter operating nearby. You might need to set another Marine band radio receiver up to verify that there is indeed a signal present on the particular Marine band channel that you're monitoring. A list of frequencies is published on pg 27 (of the manual linked above) in case you need to convert between channel and frequency to tune on a non-channelized radio.
Outside of that, there's little the end user can do without specialized test equipment, schematics and of course, training. Many shops will be happy to provide an estimate for repair when provided with a small bench diagnostic fee. This fee is usually applied towards an authorized repair by most reputable repair shops.
I hope this helps & good luck!