diff --git a/audio_test.go b/audio_test.go
index 9f32d54..51b3f46 100644
--- a/audio_test.go
+++ b/audio_test.go
@@ -2,12 +2,16 @@ package openai //nolint:testpackage // testing private field
 
 import (
 	"bytes"
+	"context"
+	"errors"
 	"fmt"
 	"io"
+	"net/http"
 	"os"
 	"path/filepath"
 	"testing"
 
+	utils "github.com/sashabaranov/go-openai/internal"
 	"github.com/sashabaranov/go-openai/internal/test"
 	"github.com/sashabaranov/go-openai/internal/test/checks"
 )
@@ -107,3 +111,131 @@ func TestCreateFileField(t *testing.T) {
 		checks.HasError(t, err, "createFileField using file should return error when open file fails")
 	})
 }
+
+// failingFormBuilder always returns an error when creating form files.
+type failingFormBuilder struct{ err error }
+
+func (f *failingFormBuilder) CreateFormFile(_ string, _ *os.File) error {
+	return f.err
+}
+
+func (f *failingFormBuilder) CreateFormFileReader(_ string, _ io.Reader, _ string) error {
+	return f.err
+}
+
+func (f *failingFormBuilder) WriteField(_, _ string) error {
+	return nil
+}
+
+func (f *failingFormBuilder) Close() error {
+	return nil
+}
+
+func (f *failingFormBuilder) FormDataContentType() string {
+	return "multipart/form-data"
+}
+
+// failingAudioRequestBuilder simulates an error during HTTP request construction.
+type failingAudioRequestBuilder struct{ err error }
+
+func (f *failingAudioRequestBuilder) Build(
+	_ context.Context,
+	_, _ string,
+	_ any,
+	_ http.Header,
+) (*http.Request, error) {
+	return nil, f.err
+}
+
+// errorHTTPClient always returns an error when making HTTP calls.
+type errorHTTPClient struct{ err error }
+
+func (e *errorHTTPClient) Do(_ *http.Request) (*http.Response, error) {
+	return nil, e.err
+}
+
+func TestCallAudioAPIMultipartFormError(t *testing.T) {
+	client := NewClient("test-token")
+	errForm := errors.New("mock create form file failure")
+	// Override form builder to force an error during multipart form creation.
+	client.createFormBuilder = func(_ io.Writer) utils.FormBuilder {
+		return &failingFormBuilder{err: errForm}
+	}
+
+	// Provide a reader so createFileField uses the reader path (no file open).
+	req := AudioRequest{FilePath: "fake.mp3", Reader: bytes.NewBuffer([]byte("dummy")), Model: Whisper1}
+	_, err := client.callAudioAPI(context.Background(), req, "transcriptions")
+	if err == nil {
+		t.Fatal("expected error but got none")
+	}
+	if !errors.Is(err, errForm) {
+		t.Errorf("expected error %v, got %v", errForm, err)
+	}
+}
+
+func TestCallAudioAPINewRequestError(t *testing.T) {
+	client := NewClient("test-token")
+	// Create a real temp file so multipart form succeeds.
+	tmp := t.TempDir()
+	path := filepath.Join(tmp, "file.mp3")
+	if err := os.WriteFile(path, []byte("content"), 0644); err != nil {
+		t.Fatalf("failed to write temp file: %v", err)
+	}
+
+	errBuild := errors.New("mock build failure")
+	client.requestBuilder = &failingAudioRequestBuilder{err: errBuild}
+
+	req := AudioRequest{FilePath: path, Model: Whisper1}
+	_, err := client.callAudioAPI(context.Background(), req, "translations")
+	if err == nil {
+		t.Fatal("expected error but got none")
+	}
+	if !errors.Is(err, errBuild) {
+		t.Errorf("expected error %v, got %v", errBuild, err)
+	}
+}
+
+func TestCallAudioAPISendRequestErrorJSON(t *testing.T) {
+	client := NewClient("test-token")
+	// Create a real temp file so multipart form succeeds.
+	tmp := t.TempDir()
+	path := filepath.Join(tmp, "file.mp3")
+	if err := os.WriteFile(path, []byte("content"), 0644); err != nil {
+		t.Fatalf("failed to write temp file: %v", err)
+	}
+
+	errHTTP := errors.New("mock HTTPClient failure")
+	// Override HTTP client to simulate a network error.
+	client.config.HTTPClient = &errorHTTPClient{err: errHTTP}
+
+	req := AudioRequest{FilePath: path, Model: Whisper1}
+	_, err := client.callAudioAPI(context.Background(), req, "transcriptions")
+	if err == nil {
+		t.Fatal("expected error but got none")
+	}
+	if !errors.Is(err, errHTTP) {
+		t.Errorf("expected error %v, got %v", errHTTP, err)
+	}
+}
+
+func TestCallAudioAPISendRequestErrorText(t *testing.T) {
+	client := NewClient("test-token")
+	tmp := t.TempDir()
+	path := filepath.Join(tmp, "file.mp3")
+	if err := os.WriteFile(path, []byte("content"), 0644); err != nil {
+		t.Fatalf("failed to write temp file: %v", err)
+	}
+
+	errHTTP := errors.New("mock HTTPClient failure")
+	client.config.HTTPClient = &errorHTTPClient{err: errHTTP}
+
+	// Use a non-JSON response format to exercise the text path.
+	req := AudioRequest{FilePath: path, Model: Whisper1, Format: AudioResponseFormatText}
+	_, err := client.callAudioAPI(context.Background(), req, "translations")
+	if err == nil {
+		t.Fatal("expected error but got none")
+	}
+	if !errors.Is(err, errHTTP) {
+		t.Errorf("expected error %v, got %v", errHTTP, err)
+	}
+}
diff --git a/config_test.go b/config_test.go
index 145c260..9602308 100644
--- a/config_test.go
+++ b/config_test.go
@@ -100,3 +100,24 @@ func TestDefaultAnthropicConfigWithEmptyValues(t *testing.T) {
 		t.Errorf("Expected BaseURL to be %v, got %v", expectedBaseURL, config.BaseURL)
 	}
 }
+
+func TestClientConfigString(t *testing.T) {
+	// String() should always return the constant value
+	conf := openai.DefaultConfig("dummy-token")
+	expected := "<OpenAI API ClientConfig>"
+	got := conf.String()
+	if got != expected {
+		t.Errorf("ClientConfig.String() = %q; want %q", got, expected)
+	}
+}
+
+func TestGetAzureDeploymentByModel_NoMapper(t *testing.T) {
+	// On a zero-value or DefaultConfig, AzureModelMapperFunc is nil,
+	// so GetAzureDeploymentByModel should just return the input model.
+	conf := openai.DefaultConfig("dummy-token")
+	model := "some-model"
+	got := conf.GetAzureDeploymentByModel(model)
+	if got != model {
+		t.Errorf("GetAzureDeploymentByModel(%q) = %q; want %q", model, got, model)
+	}
+}