Try to borrow the .acm format music files from Killap's Restoration Poject 2.3.3 or so is the latest version.
Vanilla .acm music files + few extra tracks from Restoration project, such as the music for Sulik's Tribe. Base game .acm files are 22050Hz the .mp3 or .ogg Fo2 music files are of higher quality, and i'm certain they will have a huge impact on performance while running on your pocket calculator with win98SE.
I don't remember, what specs are you runnig it on, however the best CPU that win 98SE can detect is PIII 1,5GHz and 512MB RAM single core if running on a real PC.
even if you have a better cpu like core 2 quad on your system, it will most likely be detected as Pentium III 1,5GHz
If running on a dosbox, setting dosbox.conf to max audio frequency of 22050Hz will also improove performance, although the emulated sound device must be Sound Blaster 32/AWE, it gives a huge performance hit, although it's the only card that dosbox can emulate that fully supports classic fallout games ( supports 4 simultaneus sounds at once).
setting the card to soundblaster 1 or 2 will cause hissing when there is a sound playback on a non i386 machine
As for music files original fallout/fallout 2 and Restoration Project sound files, are recorded at 22050Hz, and playing .acm files is less taxing to the system than external support from sfall of .mp3 and/or .ogg, that's for sure.
As for Killap's RP2.3.3 try to download manual installer version ( rar'ed or zip'ed files, without an installer) they should contain the music files you need, and installer version might not work on win98 SE, as You or someone mentioned in the modding section not that long ago.
Also You can try to install DirectX libraries onto win 98SE. I remember that the most latest one that i could install was an early version of Dx9b. still have it somewhere on my cds dvds from computer magazines from early 2000's it's still not enaugh to run Dx9 mode for Fallout as this requires the latest libraries 42 and 43 which are present in jun10 2010 direct X runtime, which cannot install itself onto win 98SE. although you might try to install them manually into your system by copying over the files from a newer os with full DX runtime installed.
